Continuous Scan and Repetitive Mode FT-IR Spectroscopy and Its Application in Isomeric Identification, Conformational Analysis and Photochemistry

Abstract

This chapter intends to cover the instrumentation of gas phase Fourier transform infrared spectroscopy (FT-IR), its recent advancement, and applications. The major focus have been given to the principle and data acquisition scheme of the repetitive mode measurement method of FT-IR spectrometer. The application of this spectroscopy in the isomeric identification of the methylated polycyclic aromatic hydrocarbons (MPAHs) and the conformational analysis of diols have been discussed. Furthermore, the application of the repetitive measurement mode of FT-IR combined with the UV laser in monitoring the atmospherically relevant photochemical reactions has been covered. In conclusion, this chapter briefly summarizes the current applications and discusses future applications of this technique in following drug degradation.

1. Introduction

Fourier transform infrared spectroscopy (FT-IR) is an analytical tool that is used to obtain molecular structural information based on spectral signatures of molecular vibrations. It has been employed in numerous research fields such as chemistry, chemical industries, chemical engineering, pharmaceutical industries, atmospheric chemistry, etc. Thus, it becomes a standard instrument in modern chemical and physical laboratories.

In a conventional FT-IR spectrometer, the movable mirror of the Michelson interferometer undergoes a complete movement to produce the interferogram which is Fourier transformed to obtain a spectrum. This technique is used for getting vibrational information of molecules and molecular complexes which help in identification of isomers and conformers and finding molecular interactions such as hydrogen bonding, etc. It takes a certain amount of time for the complete movement of the mirror and thus restricts the application of FT-IR to time-evolving process.

Since its discovery, it has been passed through several modifications especially to reduce the data acquisition time so that molecular changes that happened during a reaction can be investigated in-situ [1]. These modifications include the development of time-resolved FT-IR spectroscopy such as repetitive mode [2], rapid scan [3, 4], and step-scan [3, 5, 6] which covers, typically, time-resolution of 10²–10⁻⁹ s. Several commercial FT-IR spectrometers are equipped with these options. In the case of repetitive mode measurement, several spectra are recorded in a conventional manner at a particular time interval with the continuous light illumination for photolysis; a typical time resolution that can be obtained with this technique is ∼100 ms. In the case of rapid scan mode [6], it is possible to move the mirror at high speed to record the spectra in a short time of 10’s ms. It enables to record a series of spectra after laser flashes in a time-resolved way. The series of time-resolved difference spectra can be obtained by subtracting the spectrum recorded before laser excitation. In the step-scan FT-IR spectroscopy [6], data acquisition occurs in multiple steps of the movable mirror. At each step, a laser is fired to induce the reaction, and then the IR transient induced by the laser flash is recorded. Interferograms and therefore spectra are reconstructed from a series of transients corresponding to a complete series of mirror stop position [4, 6]. In the last two decays, the step-scan FT-IR spectrometer has been coupled with the imaging technique and applied in the area of biomedical science [7, 8, 9].

The infrared spectra of molecules can be measured in various states of materials - solid, liquid, and gas phases. The former two states’ measurements are common and used in routine studies, but the later mode measurement is uncommon since the experimental requirement is more for acquiring the gas phase spectra. Furthermore, in some cases, it is way difficult to get enough molecules into the gas phase, e.g., polycyclic aromatic hydrocarbons of more than four member rings have shown vapor pressure below 10⁻⁶ mmHg at room temperature. However, among these three states, the gas phase spectra provide more valuable information. This is illustrated by comparing spectra of 1,4-butanediol (HO-C₄H₈-OH) in the gas phase [10] and liquid state [11], see Figure 1a and b.

Figure 1.

Infrared spectra of 1,4-butandiol in the gas phase (a) and liquid state(b). Spectra (b) converted from % transmission mode to absorbance mode. Spectra (a) reproduced from reference [10] with permission; Copyright (2015) ACS. Spectra (b) reproduced from reference [11] with permission; Copyright (2017) RSC.

From Figure 1, we can see that above 3500 cm⁻¹ there are three IR absorption bands in the gas phase spectra of 1,4-butanediol. Among these, the band of higher frequency one is assigned to the free O-H stretching vibration, and the remaining two bands of lower frequency value are assigned to the hydrogen-bonded O-H stretching vibrations which are belonging to the two different conformers [10]. These information are hidden in the corresponding liquid state spectra, only a broad IR absorption band appeared at ∼3300 cm⁻¹; this is what you can find in most textbooks. The other methods of sampling techniques along with instrumentation have been summarized by Marwa El-Azazy [12].

This chapter means to give an overview of the following. From the above discussion, it is clear that for measuring infrared spectra of low vapor pressure compounds, one can use a long-path gas cell to improve the detection limit. Therefore, how the FT-IR spectrometer can be assembled with the gas cell and vacuum lines to investigate the vibrational spectra of molecules and molecular complexes in the gas phase. In addition, if this basic set-up is modified and coupled with the UV laser whether that can be used to monitor photochemical reactions. Furthermore, how, it can be used to obtain quantitative information on free molecules, molecules in a mixture, and reactants and photoproducts in a reaction. Discuss the applications of these methods in the gas phase vibrational spectroscopy of methylated polycyclic aromatic hydrocarbon (MPAHs), conformation analysis of diols, and monitoring the photochemical reactions of halobenzenes.

2. Instrumentation

The principle and instrumentation of FT-IR spectrometer can be found in many textbooks, including the book written by Griffiths and Haseth [13]. Nevertheless, a brief description is required to make the connectivity. The design of interferometers used for IR spectroscopy is based on the interferometer which was originally designed in 1891 by Michelson [14]. This interferometer is a device that split a beam of radiation into two beams and then recombines the two beams after a path difference has been introduced. A condition thereby created under which interference between two beams can occur. The variation of intensity of the beam emerging from the interferometer is measured as a function of the path difference called retardation which is designated as δ. The schematic diagram of Michelson interferometer, interferogram, and spectrum for monochromatic light source is shown in Figure 2 [15]. Here a constructive interference will occur only when the distance between the movable mirror and the fixed mirror (i.e., δ) is an integral multiple of λ, whereas a destructive interference will take place when δ is an integral multiple of λ/2.

Figure 2.

Optical diagram of a Michelson interferometer with the He-Ne laser running co-axial, interferogram and spectrum for a single frequency light source. The interferometer consists of four major components: A fixed mirror, a moving mirror, BS: Beamsplitter, D: Detector. LD: He-Ne laser detector. Adapted from reference [15]. Copyright (2011) Indian Institute of Science Bangalore (https://etd.iisc.ac.in/handle/2005/1252).

To obtain an interferogram, I(δ), the detector signal is digitized and recorded as a function of retardation (δ). A reference He-Ne laser is employed to monitor movement of movable mirror. The interferogram intensity of polychromatic source is mathematically described by Eq. (1):

$$I\left(\mathrm{\delta }\right)={\int }_{-\infty }^{+\infty }B\left(\mathrm{\nu }\right)cos\left(2\mathrm{\pi \nu \delta }\right)d\mathrm{\nu }$$E1

where $B\left(\nu \right)$ is the spectral intensity at wave number ν (in cm⁻¹). The Fourier transformation (FT) of $I\left(\delta \right)$ gives the single-beam IR spectrum expressed by Eq. (2):

$$B\left(\mathrm{\nu }\right)={\int }_{-\infty }^{+\infty }I\left(\mathrm{\delta }\right)cos\left(2\mathrm{\pi \nu \delta }\right)d\mathrm{\delta }$$E2

The movable mirror moves continuously in case of continuous scan mode as discuss below.

2.1 Continuous scan interferometry

The continuous-scan is preferably used for the routine static or very slow kinetic measurements. In a continuous-scan FT-IR spectrometer, the moving mirror moves continuously at a constant velocity, V(cm s⁻¹), and the optical path difference at time t(s) is given by $\delta =2\mathit{Vt}\left(\mathit{cm}\right)$. The interferogram data points are digitized at the zero crossings of the He-Ne laser signal as shown in Figure 3 [15]. The use of laser signal ensures that I(δ) is measured at precisely equal intervals of mirror positions and provides an internal wavelength calibration for every scan. Because of continuous movement of the mirror, the interferogram I(δ) changes as a function of time. The Fourier frequency (f_F in Hz) of IR light at a wave number ν (in cm⁻¹) is given by:

Figure 3.

Schematic of data collection in continuous-scan interferometer. Adapted from reference [15]. Copyright (2011) Indian Institute of Science Bangalore (https://etd.iisc.ac.in/handle/2005/1252).

$$f_F=2V\mathrm{\nu }$$E3

where V is the mirror velocity in cm s⁻¹. Typically, the slow velocities are chosen for thermal detectors; such as deuterated triglycine sulfate (DTGS), whereas, the fast velocities are chosen for fast detectors, such as a mercury cadmium telluride (MCT) or indium antimonide (InSb) detector, for routine or kinetic measurements.

2.2 Repetitive measurements with FT-IR

The modern FT-IR spectrometer has a provision for the repetitive mode measurement. The data acquisition scheme is similar to normal scan [I(δ) vs δ] which is repeated for many times depending on the requirement to follow up the reaction kinetics completely. Thereby introduced another dimension in in conventional scan mode, and thus it is a three dimensional [I(δ) vs δ and t]. Figure 4 shows the data collection scheme of repetitive mode measurement of the FT-IR. For a set of experiment, say, five interferogram (t1 - t5) correspond to the five spectra at time resolution Δt = (t₂ - t₁) and each interferogram is an average over many continuous scans as describe in Figure 3.

Figure 4.

Schematic of data collection in repetitive mode of measurements. Interferogram t1 - t5 corresponding to the spectra 1–5, respectively. Each interferogram or thereby spectra correspond to the average over many conventional spectrums.

For a particular spectral resolution, the time resolution is limited by scanning speed of movable mirror and number of co-additions, i.e., no of scans consider for the spectra. This kind of measurements can be used to monitor photochemical and thermally perturbed process without any break. The advantage of this mode measurement is that one can do photolysis and simultaneously keep recording the spectra of sample which is not refresh like in the case molecular beam technique. That means it has capability to monitor photolysis products on-line. This is like a method which can be used to study the photo- and thermo-chemical evolution within a well-defined system. Thus, this methodology is well suited for applied problems.

2.3 Variable path-length long-path gas cell

For measuring infrared spectra of molecules and/or monitoring a photochemical reaction in the gas phase, the well-known method is to couple the gas cell with the FT-IR spectrometer. However, in some cases it is difficult to obtain sufficient amount of compounds into the gas phase from its solid or liquid states for the IR spectroscopic study using a normal gas cell of ∼10 cm (typically) optical path length (OPL). Also, this is true in cases where photolysis product yield is found to be low. In such cases, where concentrations are below the detection limit with the gas cell, the common strategy is to use longer OPL (l) to improve the IR absorbance. This is in accordance with the Lambert–Beer’s law. In other words, a higher detection limit can be achieved by employing a long-path gas cell.

Figure 5a shows a schematic of the variable path-length long-path gas cell which was made according to the “white cell” principle [16]. The cell contains three internal gold-coated mirrors; two (M1 and M2) of them are adjustable attached at the top and one (M3) is fixed at the bottom of the cell called a field mirror. These adjustable mirrors are placed at the radius of curvature of the fixed mirror. At the entrance and exit point of the IR radiation two ZnSe windows are used. Outside of both the windows two adjustable transfer mirrors are used to guide the IR light from the source to the detector through the gas cell.

Figure 5.

(a) Schematic diagram of typical “white cell” [side view]; M1, M2, M3 - mirrors, numbers (1–4) indicate the direction of light. (b) Top view of field mirror (M3); representative light images or spots on this mirror are indicated by numbers.

The OPL of the cell can be varied by changing the position of the adjustable mirror. The beam comes through one window (right-hand side) and reflects back and forth between the field mirror and the adjustable mirrors of the gas cell. After every 4 passes, the beam migrates across top of the field mirror on the input side until it misses that mirror and exits through the exit window. The field mirror shows the migration of the IR beam in the gas cell. The IR beam comes in at the position marked 0 and exits at the position marked 16, see Figure 5b.

Initially, the desired OPL is set roughly by adjusting the position of the adjustable mirror with the help of red knob in Figure 5a and simultaneously counting the He-Ne laser spots on the fixed mirror. From the number of spots as recommended by manufacturer, the IR light OPL can be calculated back. Further fine-tuning of OPL can be done by adjusting mirror slightly forward or backward and looking for the maximum interferogram signal with the empty cell. Before using the multi-pass gas cell for quantitative measurements, one needs to make sure optical path lengths change precisely and follow the linearity in absorbance vs OPL plot. Figure 6 shows the results of such calibration curve obtained using naphthalene as a standard compound. This has been carried out at room temperature. A similar calibration method is applied while setting the precursor amount for the photochemical reaction studies.

Figure 6.

Observed band area (in cm⁻¹) vs optical path length (in m) for naphthalene. Adapted from reference [15] with permission. Copyright (2011) Indian Institute of Science, Bangalore (https://etd.iisc.ac.in/handle/2005/1252).

2.4 Experimental set-up for the gas phase studies

The description of the experimental set-up can be found in Ref.s [17, 18, 19, 20]. Figure 7 shows how a variable multi-pass long-path gas cell was coupled with the FT-IR spectrometer and vacuum lines. FT-IR spectrometer utilized in this set-up was equipped with a liquid-nitrogen-cooled photovoltaic mercury-cadmium-telluride (PV-LN-MCT) detector and a KBr beamsplitter. The vacuum line can be solely homemade using 1/4″ valves, tubing, and fittings from Swagelok. It is connected to the gas cell, pressure transducer, vacuum pump, Ultra-Torr for the sample tube and bulb, and a gas reservoir cylinder. The sample (solid/liquid) holder is a glass tube/bulb which is attached to the vacuum line using Ultra-Torr fitting and O-ring.

Figure 7.

Schematic diagram of FT-IR spectrometer coupled with a variable multi-pass long-path gas cell and vacuum lines.

The sample can be loaded in a glass tube or bulb depending on the state of the sample. The dissolved gases are removed by applying several cycles of freeze-pump-thaw. The sample vapor was first allowed to expand into the vacuum line and then transfer to the gas cell and mixed with carrier gas argon (Ar, UHP). This is done in cases where solid or liquid samples have sufficient vapor pressure at room temperature but need to have a temperature controlling system to raise the T during sampling and to maintain cell T during FT-IR measurements for samples having low vapor pressure at room. The spectrometer and sample compartment were continuously purged with ultra-high purity (UHP) nitrogen gas during experiments to avoid interference from moisture and carbon dioxide. Argon (Ar) gas kept in the reservoir cylinder is used for cleaning the cell as well as carrier gas.

This setup was used for investigating vibrational spectra of gaseous PAHs and MPAHs. The vapor pressure of such compounds is low at room temperature, ranging from 10⁻² to 10⁻⁷ mmHg for 2 to 7 rings system [15]. Therefore, cell needs to heat up during sampling and acquiring spectra. For very low (< 10⁻⁷ mmHg) vapor pressure compound, the recommended way is to fill the sample bulb with a solid sample and placed it inside the cell. This setup is also used for the conformational analysis of diols in the gas phase. Furthermore, this basic setup has been modified to investigate the photochemical reactions in the gas phase.

2.5 Experimental set-up for photochemical reaction studies

The repetitive mode measurement method of FT-IR was used to monitor the photochemical reaction in the gas phase. Figure 8 shows the experimental setup [21]. The gas cell shown in this setup served as a reactor in this experiment. Its borosilicate glass body was replaced with a Quartz tube to allow UV light for the photolysis. The 266 nm light was generated from the fourth harmonic (1064 nm) of a pulse Nd:YAG laser. In order to increase the photolysis efficiency, a multi-pass arrangement was made for the laser light as well using five UV reflectors. In past, a similar approach has been applied by Eberhard et al. [22]. They used a pair of rectangular UV reflectors in order to make a multi-pass arrangement across the fixed gas cell which was coupled with a step-scan time-resolved FT-IR spectrometer. In this set-up, the reflectors were placed on circular holders which were hanging on a rectangular frame. The frame has a provision for “x” and “y” movements [21]. This reflector set-up was placed parallel to the cell body to benefit six times UV light passes through the gas cell.

Figure 8.

Schematic diagram of FT-IR spectrometer coupled with the variable multi-pass long-path gas cell, vacuum lines and UV laser (ns).

Usually, the laser pulses energy is measured before entering the gas cell. However, inside the cell, the energy is lower due to absorption by cell body material and UV reflector, which is taken into account in the photolysis efficiency estimation. The amount of precursor was set after measuring its infrared spectra as a function of sample pressure so as to keep the absorbance ∼1.2. Typically, a small amount of sample vapor was loaded, as much to keep the IR absorbance value in the working range, in the gas cell for the photolysis studies. The partial pressure of sample inside the gas cell can be estimated from its infrared integrated absorbance as described in Section 3.2.

First measured the infrared spectrum of the precursor without photolysis. The precursor stability, as well as its rate of condensation, was tested prior to photolysis by measuring its infrared spectra for up to several minutes. Then the irradiation was started, and simultaneously infrared spectra were recorded. To monitor the reaction, several spectra at different photolysis time intervals have been recorded using the repeated mode measurements of FT-IR. Point to be noted, the laser light irradiation and repetitive scan measurements were started at the same time manually. The precursor spectra were subtracted with proper weighing factors from the spectra recorded during the photolysis to get the product spectra which are called difference spectra. The depletion and formation of infrared bands in the difference spectra indicate the decomposition of precursor and formation of products, respectively.

There is always a trade-off to play with spectral resolution and number of scans per spectra to achieve a maximum temporal resolution. The FT-IR, used in this set-up, required 414 ms per scan at a spectral resolution of 2 cm⁻¹. Therefore, monitoring a photochemical reaction of lifetime 100’s of ms with spectral resolution of 2 cm⁻¹ is possible with this technique.

There are certain advantages to this technique. It can measure a wide spectral range which allows the detection of many species simultaneously. Furthermore, this technique can be used to study the chemical evolution within a well-defined isolated system; which means, it is well suited for applied problems. The disadvantage of this technique is that it suffers a lack of sensitivity as compared to the technique based on molecular beam combined with laser and mass spectrometry. However, one can, to some extent, improves the sensitivity of this technique by using a long-path gas cell. This method is used to investigate the photodegradation pathways of atmospherically and industrially relevant halogenated compounds [2].

3. Quantitative analysis

3.1 Infrared intensity

The quantity of interest in the intensity calculation is the integrated absorption coefficient A (in km mol⁻¹). It can be determined theoretically using Eq. (4) [23]

$$A_i=42.254{\left|\frac{\mathrm{\partial \mu }}{{\partial Q}_i}\right|}^2,$$E4

where $\partial \mu /\partial Q_i$ are the dipole moment derivatives in D (Å amu^1/2) evaluated via analytical derivatives computed at the DFT level of theory.

On the other hand, absolute intensity (in cm⁻² atm⁻¹) can be obtained experimentally by using the Eq. (5) [24]

$$A_i=\frac{2.303\int log{\left(I_o/I\right)}_{i}d{\mathrm{\nu }}_i}{P_{i}l},$$E5

where l (in cm) is the OPL and P_i’ s are the vapor pressures. It is not possible to get the vapor pressure of PAHs or methyl- and dimethyl-PAHs by conventional pressure gauge system since they are seeded on carrier gas(Ar). Therefore, one can estimate the partial pressures (P_i) under all the bands with the help of the observed band areas and their corresponding calculated intensities by using the same equation. The average pressure obtained from all the bands were then calculated using $\mathbf{P}\mathbf{=}{\sum }_{\mathbf{i}}^{\mathbf{n}}\frac{{\mathbf{P}}_{\mathbf{i}}}{\mathbf{n}}$ where n is the total number of observed bands which was used to calculate the observed band intensities (in cm⁻² atm⁻¹). In order to get the experimental intensities in km mol⁻¹, the values in cm⁻² atm⁻¹ were multiplied by a factor of 82.056 (T/K).

3.2 Estimation of reactant and photoproducts

The amount of sample (reactant) decay and formation of new products can be measured from the integrated band area of the observed infrared spectra using Eq. (6) [25], which is basically the modified Eq. (5).

$$P_i\left(\mathit{atm}\right)=\frac{2.303\times 82.05\times T\times {10}^{-5}\times {\tau }_i}{A_i\times l},$$E6

in which, τ_i is the integrated absorbance (in cm⁻¹), $A_i$ (in km mol⁻¹) is the absorption coefficient, l (in cm) is the OPL, and T (298 K) is the experimental temperature. Typically, we chose 2–3 absorption lines of a species and averaged the derived partial pressure. In case of rotational-resolved spectra, sum of band areas over all resolved lines is assumed to be equivalent to the unresolved band area within error limit. The derived pressure errors are typically within ±20% due to the fitting errors.

3.3 Photolysis efficiency

The estimation of the photolysis yield is discussed in this section. It helps to know the efficiency of experimental set-up. In order to do that lets take photolysis of (0.43 ± 0.01) torr C₆H₅Cl at 266 nm as an example. The photolysis decay ($x$) of precursor (C₆H₅Cl) is estimated using Eq. (7)

$$x=n\left(C_6{H}_5\mathit{Cl}\right)\times \sigma \left(266\mathit{nm}\right)\times F$$E7

where, n is the amount of C₆H₅Cl (2.829 × 10¹⁷) molecules present in the laser active volume 20.3 cm⁻³, F is the laser fluence 13.181 × 10¹⁶ photon cm⁻² pulse⁻¹ [assuming over all 25% loss on measure energy 37 mJ pulse⁻¹ due to cell and UV reflectors absorption and laser active area 0.283 cm² for ∼6 mm laser light beam diameter], and σ is the absorption cross section of C₆H₅Cl (4.933 × 10⁻¹⁹ cm² molecule⁻¹) which is according to the reported ε(266 nm) = 129 cm⁻¹ L mol⁻¹ [26]. According to Eq. (1), the amount of photoexcited C₆H₅Cl or decay of it’s found to be 1.832 × 10¹⁶ molecules pulse⁻¹ in 20.3 cm³. Eventually, this many photoproducts get distributed over the cell volume 1.805 × 10³ cm³ and thus value of $x$ is estimated to be 1.015 x 10¹³ molecules cm⁻³ pulse⁻¹ which corresponds to 3.129 × 10⁻⁴ torr pulse⁻¹. The laser is operated at 10 Hz repetition rate and thus overall accumulated decay of precursor during 53 s photolysis is estimated to be 0.166 torr; that means efficiency is 38.6%. In other words, it’s clear that dropping of 0.166 torr C₆H₅Cl is the lower limit for observing major photolysis products during first 53 s. More details of this quantitative analysis method can be found in the forthcoming article [21].

4. Applications

This section is dedicated to discussing a few representative applications of the experimental set-ups presented in Figures 7 and 8. In past many system have been investigated using these set-ups: PAHs and MPAHs namely 1- and 2-methylnaphthalenes (MNs) [27]; 1,5-, 1,6-, and 2,6-dimethylnaphthalenes (DMNs) [28]; 2,4-, 2,6-, 2,7-, and 2,8-dimethylquinolines (DMQs) [25]; 1,9-, 2,4-, and 3,9-dimethylphenanthrenes (DMPs) [20]; and fluorene, 1-methylfluorene and 1,8-dimethylfluorne [19]; conformational analysis of diols [10]; hydrogen bonded oligomers of methanol [29]; and photodegradation pathways of halobenzenes [2]. But discussion will be restricted to exemplary a few systems as discussed in the subsequent sections.

4.1 Isomeric identification of methylated-PAHs

Das et al. [28] have reported the gas phase IR spectra of 1,5-DMN, 1,6-DMN, and 2,6-DMN using the technique presented in Figure 7 and assigned their vibrational modes using density functional theory (DFT) calculations. Figure 9 shows gas-phase infrared spectra of DMNs. Also, quantitative estimation has done as discussed in Section 3.1; vapor pressure of DMNs were found to be 2.1–2.5 × 10⁻² mmHg at 90°C.

Figure 9.

The infrared absorption spectra for (a) 1,5-DMN; (b) 1,6-DMN; and (c) 2,6-DMN at 0.5 cm⁻¹ resolution. Reproduced from reference [28] with permission. Copyright (2008) Elsevier.

Table 1 listed four characteristic bands of the DMNs. These bands may help in their identification in an unknown mixture. The aromatic C-H out-of-plane bending vibration is the strongest among all the bands in DMNs and is easy to identify near 800 cm⁻¹. The DMNs are distinguishable from the position of this band which appears with a clear separation in different DMNs. A set of three bands assigned for the methyl C-H symmetric and asymmetric stretching is the next set of bands that are distinct in all the DMNs. They appear clearly resolved at the high-frequency region of the IR spectrum near 2900 cm⁻¹ with moderate intensities and can be easily marked.

Mode of vibration	1,5-DMN^c	1,6-DMN^c	2,6-DMN^c
Aromatic C-H out-of-plane	783.2 (1.000)	812.8 (1.000)	808.5 (1.000)
Methyl C-H stretching	2882.4^a (0.146); 2931.8^b (0.107); 2956.9^b (0.050)	2874.8^a (0.395); 2933.3^b (0.395); 2953.8^b (0.079)	2880.0^a (0.212); 2933.9^b (0.549); 2955.2^b (0.012)

Table 1.

Comparison of observed aromatic (C-H) out-of-plane bending and methyl C-H symmetric^a and asymmetric stretching^b vibrations in DMNs.

^cThe band positions are given in cm⁻¹ and the relative intensities in parentheses.

Reproduced from reference [28] with permission. Copyright (2008) Elsevier.

Chakraborty et al. [27] have proposed a similar identification of MNs. A few aromatic C-H stretching and aromatic C-H out-of-plane bending vibrations has been identified in the experimental spectra of MNs as evidenced from reference [27]. Out of these, a few either intense or unique bands have been chosen, as listed in Table 2, for the isomeric identification.

Mode of vibration	1-MNa	2-MNa
Aromatic C-H stretching	3076.9 (0.544)	3062.9 (1.771)
Aromatic C-C stretching	1077.2 (0.113)	1644.6 (0.206)
Aromatic C-H in-plane bending	1399.7 (0.181)	1134.6 (0.151)
Aromatic C-H out-of-plane bending	978.8 (0.087); 788.9 (1.000)	952.2 (0.152); 812.1 (1.000)

Table 2.

A few characteristic and unique bands of MNs.

^a

Frequency values are in cm⁻¹. In parenthesis, relative intensities are given.

Reproduced from reference [27] with permission. Copyright (2012) Indian Association for the Cultivation of science.

In 1- and 2-MN, an intense aromatic C-H stretch has been identified at 3076.9 and 3062.9 cm⁻¹, respectively. This band is separated by 14 cm⁻¹ from one isomer to the another. Therefore, this band can be used for the isomeric identification of MN in a complex mixture. One unique band was observed at 1644.6 cm⁻¹ in 2-MN and at 1077.2 cm⁻¹ in 1-MN. These bands are unique to those isomers and are not observed in the other isomer. Another unique band has been identified in the experimental spectra of 1- and 2-MN at 1399.7 and 1134.6 cm⁻¹, respectively, for the aromatic C-H in-plane bending vibration. This band is of low intensity and clearly visible in the recorded spectra of MNs. There are two sets of bands that are identified for aromatic C-H out-of-plane bending vibrations at 978.8 and 788.9 cm⁻¹ in 1-MN and 952.2 and 812.1 cm⁻¹ in 2-MN. The first set of bands is highly intense, whereas the second set is of low intensity. These bands are clearly distinguishable for different isomers of MN. Therefore, spectral bands in the 1800–500 cm⁻¹ region will be helpful for the isomeric identification of MNs in a complex mixture. After careful inspection of the high-intensity aromatic C-H out-of-plane bending vibrations in the region 1800–500 cm⁻¹ and at the aromatic C-H stretching vibrations in the region 3200–2800 cm⁻¹, it is possible to distinguish between the MNs. The isomeric identification through infrared spectra of these compounds, as suggested here, will perhaps be relevant in the field of environmental and atmospheric chemistry.

Similarly, a few characteristic infrared absorptions have been found for other series of methylated PAHs such as DMPs and DMQs. The details assignment and isomeric identifications for these systems are beyond the scope of discussion here. Therefore, interested readers are directed to go through reference [20, 25].

4.2 Conformational analysis using steady state measurements

Das et al. [10] applied the technique presented in Figure 7 to measure the infrared spectra of 1,2-ethandiol (1,2-ED) and 1,4-butanediol (1,4-BD) in the gas phase at different T’s. The corresponding observed spectra are presented in Figures 10A and 11A′ and compared with the simulated spectra constructed based on a mixture of conformers, Figures 10B and 11B′ and done the conformational analysis.

Figure 10.

The experimental IR absorption spectra of 1,2-ED in the gas phase at 303 K (A) and stimulated spectrum of a mixture of stable conformers tGg′, gGg′, g′Gg′, tTt, tTg, gTg′, gTg, gGg, tGt, and tGg (B). B3LYP/aug-cc-pVDZ calculated anharmonic intensities in the individual spectra of conformers were weighted by the population of the respective conformer. The simulated spectrum (B) was obtained using Gaussian functions centered at the anharmonic frequencies and with a bandwidth at half-height of 35 cm⁻¹. The O-H stretching region marked with gray. Reproduced from reference [10] with permission. Copyright (2015) ACS.

Figure 11.

Experimental IR absorption spectra of 1,4-BD in the gas phase at 313 K (A′), stimulated spectrum of a mixture of conformers g′GG′Gt, gG′G′Gt, tG′TGt, g′TTGt, gGTGt, tTTTt, tGGGt, tTGG′t, gTGGt, and tTGTt (B′). B3LYP/aug-cc-pVDZ calculated anharmonic intensities in the individual spectra of conformers were weighted by the population of the respective conformer [10]. The simulated spectrum (B′) was obtained using Gaussian functions centered at the anharmonic frequencies and with a band width at half-height of 35 cm⁻¹. The O-H stretching region marked with gray. Reproduced from reference [10] with permission. Copyright (2015) ACS.

A statistical thermodynamic population analysis at experimental temperatures have done with the chosen 10 conformers of 1,2-ED (tGg′, gGg′, g′Gg′, tTt, tTg, gTg′, gTg, gGg, tGt, and tGg) and 1,4-BD (g′GG′Gt, gG′G′Gt, tG′TGt, g′TTGt, gGTGt, tTTTt, tGGGt, tTGG′t, gTGGt, and tTGTt). The purpose of the theoretical population analysis is to predict which conformers contribute to the experimentally measured vibrational spectra. The standard statistical mechanical relations were used to calculate the free energies of chosen conformers of 1,2-ED and 1,4-BD using DFT (B3LYP/aug-cc-pVDZ). Then, the fractional gas phase equilibrium population [10, 30, 31, 32], P(M) of a conformer M is calculated according to the Boltzmann distribution

$$\mathit{P}\left(\mathit{M}\right)=\frac{\mathbf{exp}\left(-\frac{∆{\mathit{G}}_{\mathit{M}}}{\mathit{RT}}\right)}{{\sum }_{\mathit{i}}\mathbf{exp}\left(-\frac{∆{\mathit{G}}_{\mathit{i}}}{\mathit{RT}}\right)}$$E8

where i spans all the conformers of 1,2-ED and 1,4-BD, respectively. The calculated P(M) at experimental temperatures were multiplied by 100 in to order to get the percentage population. It was assumed that population of each conformer corresponds to its weight at the experimental temperatures. The stimulated spectrum Figure 10B of 1,2-ED is a mixture of conformers tGg′, gGg′, g′Gg′, tTt, tTg, gTg′, gTg, gGg, tGt, and tGg with a contribution of 55.5, 22.6, 13.4, 0.5, 2.8, 1.3, 0.4, 1.0, 1.0, and 1.5%, respectively. Similarly, the populations estimated with the B3LYP/aug-cc-pVDZ method were found to be 12.8, 4.4, 21.8, 26.8, 18.8, 3.3, 1.9, 5.1, 3.9, and 1.2% for g′GG′Gt, gG′G′Gt, tG′TGt, g′TTGt, gGTGt, tTTTt, tGGGt, tTGG′t, gTGGt, and tTGTt conformers, respectively. It has been found that most stable hydrogen bonded conformers of 1,4-BD are less populated than some of the non-hydrogen bonded conformers. Even for the 1,4-BD, the relative population of the g′GG′Gt conformer [10], which has strong intramolecular hydrogen bond, is less than what is predicted. It has been proposed that perhaps the intramolecular hydrogen bond is not the only factor governing the relative stability of the hydrogen bonded conformers of diols.

This comparative study has revealed a few notable things. First of all, experimental observed spectra are a mixture of hydrogen and non-hydrogen bonded conformers of diols. Second thing is that it helps in finding what is the weighting factor by which individual conformer contributed to the measure spectra that indirectly indicating in what % each conformer exist in a mixture. Furthermore, based on this comparative study, it has been concluded that strong intramolecular hydrogen bonding exists in 1,4-BD but it appears to be weak intramolecular hydrogen bonding in 1,2-ED at temperatures of 303, 313 and 323 K in the gas phase [10].

4.3 Photochemistry with time resolved set-up

Using gas phase time-resolved FT-IR set-up described in Section 2.6, the very first system that has been studied by Behera et al. [2] is the photodegradation of chlorobenzene (C₆H₅Cl). During this measurement, spectrometer has been set to acquired 5 spectra at 120 sec intervals for a total period of about 9 min with a spectral resolution 2 cm⁻¹ and averaged over 128 scans. Figure 12a shows the spectra of precursor (C₆H₅Cl) which is obtained based on normal scan measurement, whereas spectra in Figure 12b–f, obtained during 53, 173, 293, 413, and 533 s photolysis of C₆H₅Cl were based on repetitive mode measurement.

Figure 12.

(a) Infrared absorption spectra of 0.43 ± 0.01 torr C₆H₅Cl seeded in 142 torr argon (Ar). Difference spectra (b–f) were measured during 53, 173, 293, 413, and 533 sec photolysis at 266 nm. Infrared absorption band indicated by A, B, C, and D are assigned to hydrochloric acid (HCl), acetylene (C₂H₂), 1,3-butadiyne (C₄H₂), and phenol (C₆H₅OH), respectively. Reproduced from reference [2] with permission. Copyright (2021) Elsevier.

Following 266 nm photolysis of C₆H₅Cl, the ro-vibrational lines were observed in the region 3060–2625 cm⁻¹, at 3317.8/ 3262.7 cm⁻¹ and 1346.2/1301.2 cm⁻¹, and at 3341.2 and 1232.7 cm⁻¹. These infrared features are assigned to the hydrochloric acid, acetylene, and 1,3-butadiyne, respectively. Identification of acetylene and 1,3-butadiyne but not expected HCl co-product ortho-benzyne (o-C₆H₄) indicates, possibly, o-C₆H₄ further degraded into acetylene and 1,3-butadiyne. The calculated potential energy surfaces for the possible degradation channels of C₆H₅Cl shown that HCl elimination and C-Cl bond fission are major degradation paths.

Further quantitative analysis has done using the measured infrared absorbance for the precursor and various photoproducts and Eq. (5). The results of this analysis have been presented in Figure 13.

Figure 13.

(a) Decay of chlorobenzene (C₆H₅Cl) and growth of products (b) hydrochloric acid (HCl), (c) acetylene (C₂H₂), and (d) 1,3-butadiyne (C₄H₂) as a function of 266 nm photolysis time. The plot is based on repetitive mode measurements, see Figure 12 and Eq. (5) in the main text. Reproduced from reference [2] with permission. Copyright (2021) Elsevier.

Accordingly, the initial pressure of precursor is estimated to be 0.427 ± 0.013 torr. During first 53 s photolysis the concentration of C₆H₅Cl is decreased by (0.0128 ± 0.241 × 10⁻³) torr and formation of primary product HCl is estimated to be (0.00645 ± 0.19894 × 10⁻⁴) torr as reflected in Figure 13. This clearly indicates that, the experimental branching ratio for the HCl elimination and C-Cl bond fission paths of C₆H₅Cl is 1:1. This is consistent with the calculate energetics which indicates that the energy required for the C-Cl bond fission is closed to the HCl elimination barrier of C₆H₅Cl. Further details assignment of photodegraded products and reaction mechanism can be found in recent publication [2].

This gas phase time resolved technique was successfully applied to investigate the photodegradation of C₆H₅Br at 266 nm.

5. Conclusions

These studies have provided an experimental verification of low concentration infrared spectroscopic measurements using multi-pass long-path gas cell coupled with the FT-IR spectrometer. This is essential for the detection of atmospherically and astronomically important PAHs and MPAHs. The low concentration measurement is also essential for the detection of intramolecular hydrogen bonded conformers in a gaseous mixture. Furthermore, even its required for the detection of photoproducts in a mixture of reactant and products.

The benchmark data on vibrational signatures of some of the MPAHs are expected to help in identification of them in a complex mixture from various sources. These studies also provide the spectroscopic method of quantification of gaseous MPAHs which will eventually help in quantification of them so that worldwide emission rate and adverse effects on human being can be monitored.

The gas phase infrared spectra are expected to provide more information as compared to solid or liquid state spectra. This has been inferred based on comparison of gas phase spectra and its corresponding liquid phase spectra of diols. Also, experimental spectra can be reconstructed after properly weightage of calculated spectrum for various conformers which helps in finding % contribution of various conformers of diols in a complex mixture. Perhaps, this is something, similar to the method called multivariate analysis.

The repetitive mode time resolved FT-IR coupled with laser was successful to probe the HCl elimination channel in the UV photolysis of C₆H₅Cl combined with ab initio calculations. The secondary products C₂H₂ and C₄H₂ was observed in conjunction with the primary product HCl. These secondary products play important roles to obscure the results from the primary processes particularly when low time resolution measurement was involved. The experimental conditions like continuous photolysis of chlorobenzene seeded in a buffer gas mimic the reaction in the atmosphere.

The future potential application such technique is in the pharmaceutical industries to find the drug degradation. The study of the photodegradation behavior of pharmaceuticals in the environment is a key issue in terms of the formation of toxic products. Therefore, information on drug degraded products is crucial to understand the environmental fate of contaminants and to establish the important degradation pathways. This can be done using this low time-resolved FT-IR spectroscopy.

Acknowledgments

My sincere thanks to Prof. Puspendu K. Das and Prof. E. Arunan of the Department of Inorganic and Physical Chemistry (IPC), Indian Institute of Science (IISc), Bangalore, for allowing us to use their lab and departmental facilities. My sincere thanks to Dr. Shubhadip Chakraborty and Dr. Bedabyas Behera for their contribution of some part of their work as cited. The spectrometer used in the experiment was supported by the FIST program of the Department of Science and Technology, Govt. of India. Dr. Das thank Center for Advanced Research Studies, Ganpat University (CARS-GUNI) for providing working platform. Also, thanks to all authors for given permission to use their published work either in the form of data or figure.

Conflict of interest

The authors declare no conflict of interest.