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The Zwitterionic property of aminoacids give molecular crystal formation through homodesmotic reaction with smaller organic molecules which can undergo hydrogen bonding interactions. Alpha hydroxyl phenyl acetic acid known as mandelic acid (MA) was added with essential amino acid, L-phenylalanine (LPA) resulted in the formation of molecular crystal with P21 space group otho rhombic crystal containing four units (namely one MA, two LPA and one water) bis-L –phenyl alanine mandelate (BLPAMA) by slow evaporation method. The single crystal obtained was subjected to characterisation studies. Recrystallised BLPAMA using methanol, subjected to slow evaporation method resulted in the formation of non centerosymmetric C2 point group monoclinic single crystal of R-phenylalanine-S-mandelate (RPASMA) confirmed with XRD study. The theoretical DFT study of RPASMA using Gaussian 09 software to study the non-covalent interactions with MO6,6-31++G(d,p) showed encouraging results for the formation of low energy gap, highly reactive RPASMA. The H-bonding in the crystal confirmed by DFT study showed the existence of three units – MA, H and LPA in the crystal. Compared the experimental and theoretical crystal parameters of the reactants (MA, LPA) and product (RPASMA) for the thermo chemical properties, intermolecular hydrogen bonding existing between MA and LPA stabilises the structure of the formed RPASMA crystal resulting in the small difference in energy gap observed from HOMO-LUMO studies indicate the highly reactive character of RPASMA.
Department of Chemistry, Sri Sairam Engineering College, Chennai, India
Charles Kanakam Christopher
Department of Chemistry, Presidency College, Chennai, India
*Address all correspondence to: usha.che@sairam.edu.in
1. Introduction
MA has a long history of use in the medical community as an antibacterial, particularly in the treatment of urinary tract infections. It has also been used as an oral antibiotic and as a component of chemical face peels analogous to other alpha hydroxy acids. LPA is an essential aromatic amino acid in humans (provided by food), LPA plays a key role in the biosynthesis of other amino acids and is important in the structure and function of many proteins and enzymes. LPA is converted to tyrosine, used in the biosynthesis of dopamine and norepinephrine neurotransmitters. The LPA is incorporated into proteins, while the D-form acts as a painkiller. Absorption of ultraviolet radiation by Phenylalanine (PA) is used to quantify protein amounts.
LPA is the L-enantiomer of PA. It has a role as a nutraceutical, a micronutrient, an Escherichia coli metabolite, a Saccharomyces cerevisiae metabolite, a plant metabolite, an algal metabolite, a mouse metabolite, a human xenobiotic metabolite and an EC 3.1.3.1 (alkaline phosphatase) inhibitor. It is an erythrose 4-phosphate/phosphoenolpyruvate family amino acid, a proteinogenic amino acid, a phenylalanine and a L-alpha-amino acid. LPA is a conjugate base of a L-phenylalaninium, conjugate acid of a L-phenylalaninate, an enantiomer of a D-phenylalanine and a tautomer of a L-phenylalanine zwitterion.
Homodesmic reaction of MA and LPA results in the two/four components molecular single crystals due to the zwitter ionic form of phenyl alanine. The formation of non linear crystals having low energy gap highly reactive phenylalanine mandelates are stabilised due to the intermolecular H-bonding and intramolecular hydrogen bonding along with the van der Waals forces of attraction between the molecules in the crystal formation [1, 2]. The hydrogen bonding interactions induces the polarisability character and dipole moments of the compounds are resulting in the application of the compounds in different areas like opto electronics property, biological activity, antioxidant activity etc., [3, 4, 5]. The experimental solid state measurements are compared with the theoretical gaseous state measurement for the crystal parameters, FTIR, FT Raman, thermochemical properties to confirm the applications of the product and the reactants. Nonlinear nature of the compounds comply with the expected vibrational frequencies calculated using the formula (3N-6) with the theoretical vibrational changes contribution to Total Energy Distribution (TED) [6]. The charge transfer mechanism in the molecule and the presence of high antioxidant power and antiradical power of RPASMA is confirmed by Homo-Lumo small energy gap, electrochemical Cyclic Voltametric analysis [7, 8].
The compounds MA and LPA were taken to obtain the two component molecular crystal RPASMA by two steps mentioned below.
2.1 Step – 1: Synthesis of BLPAMA
Alfa Aesar DL- Mandelic acid(99%) and Nice chemicals L-phenylalanine in the molar ratio 1:2 respectively were taken in a beaker, dissolved in water, stirred well in a magnetic stirrer and obtained the molecular crystal BLPAMA by slow evaporation at room temperature. Confirmed the crystal structure by characterisation studies.
2.2 Step – 2: Synthesis of RPASMA
BLPAMA crystals were dissolved in 1:10 mmol in AR grade methanol, filtered, a clear pale yellow solution was obtained. Filterate was allowed to slow evaporation at RT, resulted in yellow coloured molecular crystals and confirmed the structure by characterisation studies.
Non-centerosymmetric crystal information of RPASMA given in CCDC number 1452128.
Gaussian 09 with MO6 DFT 6-31++G(d,p) basis set shows the existence of hydrogen bonding charge transfer mechanism in the molecular crystal formation resulting in the formation of non-centerosymmetric structure having C2 space group [9]. Experimental parameters in the solid crystalline state XRD is compared with the gaseous phase density functional measurement using Gaussian 09 with MO6 DFT 6-31++G(d,p) basis sets are compared and discussed in detail [10]. The optimised DFT structure of the compounds are shown in the Figures 1–3.
Figure 1.
Labelled MA.
Figure 2.
Labelled LPA.
Figure 3.
Optimised labelled structure of RPASMA using MO6 DFT with 6-31++G(d,p).
The interactions in the title compounds, types of electrons, protons, vibrations are compared and discussed in detail [11]. Comparison of thermo chemical properties of the compounds show the energy involved during the crystal formation [12]. The electron transfer from the LPA molecule Highest occupied molecular orbital (HOMO) interaction with the lowest unoccupied orbital (LUMO) of MA molecule is confirmed from the theoretical DFT study supports the experimental study of the compounds [13].
As the size of the molecule increases the charge and dipole moment increase and hence polarisability increases as shown in Table 1.
Particulars
MADFT.out
LPADFT.out
RPASMADFT.out
File Type
.log
.log
.log
Calculation Type
FREQ
FREQ
FREQ
Calculation Method
RM06
RM06
RM06
Charge
0
0
0
Spin
Singlet
Singlet
Singlet
Solvation
None
None
None
E(RM06) Hartree
−535.04918
−554.4665
−1089.5313
RMS Gradient Norm Hartree/Bohr
2.14E-05
8.30E-06
1.59E-05
Imaginary Freq
0
0
0
Dipole Moment Debye
3.2937709
4.924759
7.0780304
Polarizability a.u.
100.668
117.08567
215.24667
Table 1.
Comparison of DFT details of MA, LPA and RPASMA using G09 MO6-31++G(d,p).
The compound RPASMA formed through donor-acceptor charge transfer mechanism during noncenterosymmetric monoclinic molecular crystal formation results in the variation of bond lengths, bond angles, dihedral angles and torsional angles than expected values. The hydrogen bonding interactions give distorted noncenterosymmetric structure during crystallisation. The XRD values and the DFT values are compared for the compounds MA, LPA and RPASMA and are reported in detail.
4.1 Bond length
The expected average C-C bond length is 1.54 Å, C-O bond length is 1.43 Å, C-N bond length is 1.43 Å, C-H bond length is 1.09 Å, C=O bond length is 1.23 Å, O-H bond length is 1.67 Å, N-H bond length is 2.1 Å. The comparative bond length study of the compounds show the C-C, C-H, C-O and O-H the equal XRD and DFT values shown in the Table 2. The hydrogen bonding existing in C-O, O-H, N-H, C-H shows considerable variation in values in RPASMA for the respective atoms of MA and LPA. N1-H9 bond length in LPA theoretical value is higher than experimental value due to the measurement in gaseous state and solid state and the zwitterionic nature of amino acid [14].
Experimental
Theoretical
Corresponding atoms in RPASMA
Experimental
Theoretical
Atoms in MA
C1-C2
1.507
1.519
C7-C8
1.521
1.525
C1-O1
1.298
1.347
C8-O1
1.201
1.223
C1-O2
1.197
1.203
C8-O2
1.298
1.308
C2-H1
0.979
1.103
C7-H7
0.981
1.101
C2-C3
1.505
1.514
C6-C7
1.516
1.514
C2-O3
1.407
1.402
C7-O3
1.409
1.407
C3-C4
1.366
1.397
C6-C5
1.381
1.395
C3-C8
1.359
1.394
C6-C1
1.368
1.395
C4-H5
0.93
1.088
C5-H5
0.93
1.091
C4-C5
1.369
1.391
C4-C5
1.378
1.392
C5-H4
0.929
1.087
C4-H4
0.931
1.089
C5-C6
1.344
1.394
C4-C3
1.361
1.394
C6-H3
0.931
1.087
C3-H3
0.931
1.088
C6-C7
1.329
1.392
C3-C2
1.371
1.393
C7-H2
0.931
1.087
C2-H2
0.931
1.088
C7-C8
1.378
1.392
C2-C1
1.382
1.392
C8-H6
0.929
1.089
C1-H1
0.931
1.088
01-H7
0.819
0.971
O1-H1D
0.811
0.964
O3-H8
0.819
0.964
O3-H3A
0.811
1.011
Atoms in LPA
C1-C2
1.519
1.533
C17-C16
1.532
1.557
C1-O1
1.227
1.205
C17-O4
1.251
1.269
C1-O2
1.251
1.336
C17-O5
1.229
1.227
C2-H1
0.981
1.102
C16-H16
0.981
1.097
C2-C3
1.519
1.539
C16-C15
1.527
1.523
C2-N1
1.486
1.465
C16-N1
1.483
1.506
C3-H2
0.969
1.099
C15-H15A
0.971
1.096
C3-H3
0.971
1.098
C15-H15B
0.971
1.101
C3-C4
1.499
1.507
C15-C14
1.502
1.505
C4-C5
1.373
1.398
C14-C13
1.383
1.401
C4-C9
1.376
1.397
C14-C9
1.381
1.395
C5-H4
0.931
1.091
C13-H13
0.931
1.089
C5-C6
1.369
1.392
C13-C12
1.381
1.391
C6-H5
0.931
1.088
C12-H12
0.931
1.088
C6-C7
1.368
1.392
C12-C11
1.361
1.393
C7-H6
0.931
1.088
C11-H11
0.931
1.088
C7-C8
1.359
1.392
C11-C10
1.356
1.391
C8-H7
0.929
1.088
C10-H10
0.931
1.088
C8-C9
1.375
1.392
C10-C9
1.377
1.392
C9-H8
0.931
1.088
C9-H9
0.931
1.092
N1-H9
0.895
1.959
N1-H1A
0.891
1.052
N1-H10
1.006
1.014
N1-H1B
0.941
1.025
N1-H11
0.884
1.018
N1-H1C
0.881
1.021
Table 2.
Bond length (Å).
4.2 Bond angle (Å)
The intermolecular hydrogen bonding during the crystallisation of RPASMA give variations in bond angles due to sp3 hybridisation and sp2 hybridisation compounds because of their measurements in solid phase XRD and gaseous phase DFT are shown in the Table 3 and theoretical values are due to the intramolecular hydrogen bonding and the respective measurements in the RPASMA are due to the internal strain and stress factors experienced during inter molecular hydrogen bonding. In the experimental and theoretical measurements of LPA show variation in bond angles due to the zwitter ionic nature of amino acid and the respective atoms in RPASMA show variation in the atoms involved in inter molecular hydrogen bonding which causes strain in the crystallisation [15].
Experimental
Theoretical
Corresponding atoms in RPASMA
Experimental
Theoretical
Atoms in MA
C2-C1-O1
111.576
110.029
C7-C8-O1
104.651
106.701
C2-C1-O2
124.451
126.239
C7-C8-O2
122.801
122.141
O1-C1-O2
123.961
123.677
O1-C8-O2
124.361
126.171
C1-C2-H1
108.204
107.312
C8-C7-H7
109.101
106.451
C1-C2-C3
112.203
108.162
C8-C7-C6
111.431
110.231
C1-C2-O3
108.211
107.248
C8-C7-O3
112.801
111.681
H1-C2-C3
108.221
108.863
H7-C7-C6
109.101
108.771
H1-C2-O3
108.192
111.824
H7-C7-O3
109.045
111.395
C3-C2-O3
111.681
113.194
C6-C7-O3
113.482
113.073
C2-C3-C4
120.832
119.431
C7-C6-C5
119.401
120.101
C2-H1-C8
121.411
120.819
C7-H7-C1
121.601
120.281
C4-C3-C8
117.744
119.746
C5-C6-C1
119.001
119.611
C3-C4-H5
119.749
119.095
C6-C5-H5
119.901
120.261
C3-C4-C5
120.427
119.984
C6-C5-C4
120.301
120.281
H5-C4-C5
119.824
120.921
H5-C4-C5
119.901
120.261
C4-C5-H4
119.497
119.814
C4-C5-H4
119.701
120.141
C4-C5-C6
120.937
120.141
C4-C5-C3
120.501
120.001
H4-C5-C6
119.566
120.045
H4-C5-C3
119.421
120.103
C4-C5-H3
120.252
120.034
C5-C3-H3
109.053
108.774
C4-C5-C7
119.411
119.961
C5-C3-C2
111.424
108.774
H3-C5-C7
120.336
120.005
H3-C3-C2
120.301
120.121
C5-C7-H2
119.635
120.178
C3-C2-H2
119.801
120.041
C5-C7-C8
120.662
119.957
C3-C2-C1
120.501
120.201
H2-C7-C8
119.704
119.864
H2-C2-C1
119.801
119.761
C3-C8-C7
120.802
120.209
C6-C1-C2
120.201
120.071
C3-C8-H6
119.631
119.517
C6-C1-H1
119.901
119.101
C7-C8-H6
119.568
120.274
C2-C1-H1
119.901
120.841
C1-O1-H7
109.441
107.042
C8-O1-H1D
114.798
108.093
C2-O3-H8
109.459
108.257
C7-O3-H3A
107.024
112.549
Atoms in LPA
C2-C1-O1
118.488
123.816
C16-C17-O4
118.401
113.841
C2-C1-O2
116.555
113.748
C16-C17-O5
117.201
115.261
O1-C1-O2
124.906
122.421
O4-C17-O5
124.301
130.901
C1-C2-H1
109.025
104.789
C17-C16-H16
107.701
108.391
C1-C2-C3
111.507
112.325
C17-C16-C15
111.201
113.161
C1-C2-N1
109.834
108.267
C17C16-N1
110.571
107.111
H1-C2-C3
109.055
108.249
H16-C16-C15
107.701
110.161
H1-C2-N1
109.029
106.434
H16-C16-N1
107.701
106.891
C3-C2-N1
108.351
116.027
C15-C16-N1
111.701
110.891
C2-C3-H2
109.061
107.115
C16-C15-H15A
108.401
105.381
C2-C3-H3
109.016
109.336
C16-C15-H15B
108.401
110.091
C2-C3-C4
113.019
113.935
C16-C15-C14
115.381
112.841
H2-C3-H3
107.729
107.264
H15A-C15-H15B
107.501
107.301
H2-C3-C4
108.927
108.417
H15A-C15-C14
108.401
111.031
H3-C3-C4
108.953
110.504
H15B-C15-C14
108.401
109.991
C3-C4-C5
120.524
119.383
C15-C14-C13
120.801
120.521
C3-C4-C9
121.631
121.944
C15-C14-C9
121.601
120.511
C5-C4-C9
117.834
118.666
C13-C14-C9
117.601
118.931
C4-C5-H4
119.346
119.315
C14-C13-H13
119.501
119.321
C4-C5-C6
121.301
120.995
C14-C13-C12
120.901
120.251
H4-C5-C6
119.354
119.687
H13-C13-C12
119.501
120.421
C5-C6-H5
120.147
119.944
C13-C12-H12
119.901
119.711
C5-C6-C7
119.766
119.872
C13-C12-C11
120.201
120.371
H5-C6-C7
120.087
120.182
H12-C12-C11
119.901
119.921
C6-C7-H6
119.976
120.178
C12-C11-H11
120.201
120.061
C6-C7-C8
120.087
119.598
C12-C11-C10
119.701
119.771
H6-C7-C8
119.936
120.224
H11-C11-C10
120.201
120.161
C7-C8-H7
120.041
120.024
C11-C10-H10
119.701
120.111
C7-C8-C9
119.768
120.411
C11-C10-C9
120.501
119.871
H7-C8-C9
120.191
119.565
H10-C10-C9
119.701
120.021
C4-C9-C8
121.223
120.458
C14-C9-C10
121.101
120.801
C4-C9-H8
119.384
119.035
C14-C9-H9
119.501
119.391
C8-C9-H8
119.393
120.404
C10-C9-H9
119.501
119.811
C2-N1-H9
112.675
82.203
C16-N1-H1A
113.501
112.231
C2-N1-H10
110.662
111.593
C16-N1-H1B
107.801
107.301
C2-N1-H11
109.646
111.219
C16-N1-H1C
112.701
113.631
H9-N1-H10
105.982
133.369
H1A-N1-H1B
109.001
102.361
H9-N1-H11
106.483
107.774
H1A-N1-H1C
105.001
109.561
H10-N1-H11
111.294
107.603
H1B-N1-H1C
108.001
111.161
Table 3.
Bond angle (Å).
4.3 Dihedral angle
Dihedral angle value increases in the RPASMA compared to MA and LPA, it shows decrease in value where the hydrogen bonding takes place. The negative values in the case of molecular crystal is due to the formation non-centerosymmetric organic salt having delocalisation of charge. The steric hindrance give distorted ring structure is shown in the Table 4. The decrease in torsional values of salt compared to the MA and LPA shows the high symmetry attained during crystallisation due to hydrogen bonding interactions. The charge transfer mechanism during crystallisation result in negative values [16].
Experimental
Theoretical
Corresponding atoms in RPASMA
Experimental
Theoretical
Atoms in MA
O1-C1-C2-C3
59.381
81.265
O1-C8-C7-C6
69.900
94.817
O1-C1-C2-O3
−176.954
−156.333
O1-C8-C7-O3
−167.100
−167.106
O2-C1-C2-C3
−121.821
−96.124
O2-C8-C7-C6
−110.800
−110.852
O2-C1-C2-O3
1.844
26.279
O2-C8-C7-O3
12.200
38.300
C1-C2-C3-C4
69.561
73.525
C8-C7-C6-C5
−70.900
−70.832
O3-C2-C3-C4
−52.141
−45.164
O3-C7-C6-C5
171.300
150.782
O2-C2-C3-C8
126.482
135.629
O3-C7-C6-C1
−6.900
−28.943
C8-C3-C4-C5
−0.900
−0.560
C1-C6-C4-C5
−1.600
0.480
C2-C3-C8-C7
−177.848
−179.502
C7-C6-C1-C2
179.200
178.505
C3-C4-C5-C6
1.185
0.425
C6-C4-C5-C3
1.300
0.561
C4-C5-C6-C7
−1.347
−0.024
C4-C5-C3-C2
−0.300
−0.862
C6-C5-C8-C3
−1.014
0.103
C3-C2-C1-C6
0.100
0.924
Atoms in LPA
O1-C1-C2-C3
−91.018
−24.481
O5-C17-C16-C15
−75.300
−54.612
O1-C1-C2-N1
148.858
−153.905
O5-C17-C16-N1
160.000
−177.121
O2-C1-C2-C3
86.524
156.910
O4-C17-C16-C15
103.200
125.749
O2-C1-C2-N1
−33.601
27.485
O4-C17-C16-N1
−21.500
3.240
C1-C2-C3-C4
70.644
88.972
C17-C16-C15-C14
−56.400
−63.318
N1-C2-C3-C4
−168.363
−145.745
N1-C16-C15-C14
67.600
57.061
C2-C3-C4-C5
81.910
90.273
C16-C15-C14-C9
−84.500
−102.871
C2-C3-C4-C9
−99.128
−88.697
C16-C15-C14-C13
94.900
74.611
C9-C4-C5-C6
1.012
0.228
C13-C14-C9-C10
−0.300
−0.811
C3-C4-C9-C8
−179.174
178.940
C15-C14-C13-C12
−179.600
−176.669
C5-C4-C9-C8
−0.185
−0.037
C9-C14-C13-C12
−0.200
0.853
C4-C5-C6-C7
−1.703
−0.263
C14-C9-C10-C11
0.500
0.359
C5-C6-C7-C8
1.546
0.105
C9-C10-C11-C12
−0.200
0.060
C6-C7-C8-C9
−0.736
0.083
C10-C11-C12-C13
−0.300
−0.014
C7-C8-C9-C4
0.055
−0.117
C11-C12-C13-C14
0.500
−0.449
Table 4.
Dihedral angle (Å).
4.4 Bond alteration coefficient
The Bond Alteration Coefficient (BAC) of the compounds show the solid state experimental measurements of MA and RPASMA are almost equal but the theoretical gaseous phase measurements are varying due to van der Waals forces of attraction. The BAC of LPA and RPASMA are showing variations in both solid and gaseous phase measurements indicate the zwitter ionic nature of LPA leads to charge transfer mechanism as shown in the Table 5.
Exp
BAC
DFT
BAC
RPASMA
Exp
BAC
DFT
BAC
MA
C1-C2
1.507
1.519
C17-C16
1.521
1.525
C2-C3
1.505
0.002
1.514
0.005
C16-C15
1.516
0.005
1.514
0.009
C3-C4
1.366
0.139
1.397
0.117
C15-C14
1.381
0.135
1.395
0.119
C4-C5
1.369
−0.003
1.391
0.006
C14-C13
1.378
0.003
1.392
0.003
C5-C6
1.344
0.025
1.394
−0.003
C13-C12
1.361
0.017
1.394
0.002
0.166
0.128
0.16
0.133
LPA
C4-C5
1.376
1.397
C13-C14
1.383
1.401
C5-C6
1.369
0.007
1.392
0.005
C12-C13
1.381
0.002
1.391
0.01
C6-C7
1.368
0.001
1.392
0
C11-C12
1.361
0.02
1.393
−0.002
C7-C8
1.359
0.009
1.392
0
C10-C11
1.356
0.005
1.391
0.002
C8-C9
1.375
0.016
1.392
0
C9-C10
1.377
0.021
1.392
−0.001
0.033
0.005
0.048
0.012
Table 5.
Bond alteration coefficient analysis of the title compounds.
4.5 Hydrogen bonding
The hydrogen bonding in XRD values and DFT values confirm the charge transfer mechanism happening between donor- acceptor interactions resulting in the formation non centerosymmetric crystallisation [17] as shown in the Table 6.
D – H….. A
d(D-H)
d(H…..A)
d(D…..A)
∠DHA
Expt.
Theo.
Expt.
Theo.
Expt.
Theo.
Expt.
Theo.
C(15) – ….. O(5)
0.970
1.096
2.877
2.597
3.072
2.928
92.27
96.32
O(3) – H(3A)….. O(4)
0.812
1.011
1.807
1.606
2.605
2.607
167.15
116.70
N(1) – H(1A)….. O(2)
0.886
1.052
2.057
1.747
2.918
2.744
163.59
156.50
N(1) – H(1B)….. O(4)
0.940
1.025
2.819
2.213
2.704
2.529
73.32
95.68
Table 6.
Hydrogen bonding in RPASMA.
4.6 Mulliken atomic charges for MA, LPA and RPASMA
Mulliken atomic charges of the compounds show large positive charges of the acceptor hydrogen atoms and high negative charges of the donor O atom and N atoms respectively. MA and RPASMA show decrease in charge values respectively but the carbon atoms close to hydrogen bonding show increase in the atomic charge value. H1A of RPASMA connected to nitrogen atom show higher positive charge compared to LPA because of charge transfer interactions during hydrogen bonding [18]. The Mulliken atomic charges show the presence of charge transfer mechanism in the crystal formation as shown in the Table 7.
MA ATOM
Charge
Corresponding atoms in RPASMA
Charge
LPA ATOM
Charge
RPASMA ATOM
Charge
C1
0.246
C8
−0.267
C1
0.405
C17
0.108
C2
0.13
C7
0.744
C2
−0.518
C16
−0.176
C3
−0.648
C6
−1.259
C3
0.079
C15
0.055
C4
0.699
C5
0.094
C4
−0.082
C14
−0.544
C5
−0.114
C4
−0.219
C5
−0.107
C13
0.214
C6
0.045
C3
−0.001
C6
−0.112
C12
−0.004
C7
−0.246
C2
0.006
C7
0.002
C11
−0.289
C8
−0.419
C1
0.19
C8
−0.297
C10
−0.034
H1
0.137
H7
0.132
C9
−0.016
C9
0.081
H2
0.144
H2
0.132
H1
0.199
H16
0.171
H3
0.112
H3
0.134
H2
0.147
H15A
0.217
H4
0.107
H4
0.129
H3
0.164
H15B
0.107
H5
0.109
H5
0.128
H4
0.075
H13
0.164
H6
0.094
H1
0.137
H5
0.1
H12
0.137
H7
0.4
H1D
0.385
H6
0.098
H11
0.047
H8
0.388
H3A
0.556
H7
0.104
H10
0.131
O1
−0.416
O1
−0.407
H8
0.15
H9
0.081
O2
−0.375
O2
−0.339
H9
0.416
H1A
0.506
O3
−0.393
O3
−0.333
H10
0.335
H1B
0.403
H11
0.332
H1C
0.395
O1
−0.469
O4
−0.641
O2
−0.395
O5
−0.429
N1
−0.61
N1
−0.642
Total charge
0
−0.058
0
0.058
Table 7.
Mulliken atomic charges for MA, LPA and RPASMA.
4.7 Proton NMR
The non centerosymmetric structure of the RPASMA show the chemical shifts for the various type of protons as expected for both shielding effect and deshielding effect as shown in the Table 8. The OH proton present in the H-C-OH environment and the COOH environment close to each other. The molecular strain in the molecule due to hydrogen bonding leads to the values for both experimental and theoretical values in the measurements of XRD and DFT respectively as shown in the Figures 4 and 5.
Hydrogen atom number
Position in RPASMA
Experiment (ppm)
Theoretical (ppm)
H1
MA aromatic ring
7–8
9.65
H2
MA aromatic ring
7–8
7.98
H3
MA aromatic ring
7–8
7.47
H4
MA aromatic ring
7–8
7.57
H5
MA aromatic ring
7–8
9.5
H1D
H in H-C-OH in MA
5
6.13
H7
H in OH in H-C-OH in MA
3.7
2.9
H3A
H in COOH in MA
7–8
9.7
H9
LPA aromatic ring
7–8
7.64
H10
LPA aromatic ring
7–8
8.26
H11
LPA aromatic ring
7–8
6.14
H12
LPA aromatic ring
7–8
7.99
H13
LPA aromatic ring
7–8
9.7
H15A
CH2 in LPA
7–8
4.32
H15B
CH2 in LPA
3.7
4.31
H16
H in C connected to N in LPA
3.8
4.1
H1A
H in NH3+ in LPA connected to MA
5.1
7.63
H1B
H in NH3+ in LPA
5.1
7.5
H1C
H in NH3+ in LPA
5.1
7.2
Table 8.
Proton NMR.
Figure 4.
Proton NMR of RPASMA.
Figure 5.
Carbon NMR of RPASMA.
4.8 Carbon NMR
Carbon NMR calculated to TMS show similar values for both experimental and theoretical values of chemical shifts, Table 9. The experimental values are less compared to the theoretical values because of the method of estimation in the solid state and gaseous state respectively. Aromatic carbon shifts show the increase in theoretical values, the functional group carbon atoms show almost same or decrease in theoretical values may be due to the charge delocalisation in solid and gaseous phases respectively.
Carbon atom number
Position in RPASMA
Experiment (ppm)
Theoretical (ppm)
C1
MA aromatic ring
127.08
143.97
C2
MA aromatic ring
127.97
137.71
C3
MA aromatic ring
128.51
145.51
C4
MA aromatic ring
128.85
141.83
C5
MA aromatic ring
129.88
139.73
C6
MA aromatic ring connected to C7
129.88
164.08
C7
H-C-OH in MA
72.93
73.28
C8
COOH in MA
174.51
157.17
C9
LPA aromatic ring
127.08
122.26
C10
LPA aromatic ring
127.97
136.26
C11
LPA aromatic ring
128.51
144.24
C12
LPA aromatic ring
128.85
142.21
C13
LPA aromatic ring
129.88
145.84
C14
LPA aromatic ring
129.88
154.76
C15
LPA -CH2
129.88
146.33
C16
LPA -C-N
36.99
60.94
C17
COOH in LPA
140.96
120.39
Table 9.
Comparative carbon NMR data of RPASMA.
4.9 FTIR
The non linear aromatic molecules obey the 3N-6 rule for fundamental modes of vibrations, (N is the total number of atoms in the molecule). The number of atoms in MA are 19 and the fundamental modes of vibrations expected are 51, also confirmed by DFT study. The presence of total number of 23 atoms in LPA show 63 fundamental modes of vibrations by FTIR, which has been confirmed by DFT study. The RPASMA molecule with 42 atoms which shows 120 modes of fundamental vibrations confirmed by DFT study. The comparative FTIR PLOT of the title compounds is shown in the Figure 6.
Figure 6.
Comparison of FTIR of the compounds.
4.10 FT Raman
The polarisation effect leading to dipole change is measured using FT Raman. The charge transfer mechanism in the RPASMA is confirmed by the FT Raman study and is shown in the Figure 7 and the respective values are given in the Table 10.
Figure 7.
Comparison of the compounds FT Raman study.
Wavenumber (cm−1)
MA
LPA
RPASMA
3060
0.092
0.110
0.081
2939
0.033
0.065
0.029
1604
0.033
0.032
0.028
1189
0.022
0.012
0.017
1032
0.017
0.013
0.016
1000
0.055
0.059
0.048
861
0.016
0.014
0.010
822
0.002
0.013
0.009
757
0.016
0.013
0.008
617
0.010
0.008
0.008
Table 10.
Comparison of Raman intensities of MA, LPA and RPASMA.
The thermochemical properties of the compounds indicate the spontaneity of the reaction and it is accompanied by the decrease in free energy. As the polarisation increases due to charge transfer mechanism, donor – acceptor interactions, the dipole moment increases in the compound after crystallisation as bimolecular single crystal (Table 11).
Parameter
MA
LPA
RPASMA
Total Energy (E) Hartree
−535.04918
−554.4665
−1089.5313
Zero point Vibrational Energy Kcal/Mole
93.1878
119.2589
213.9199
Dipole moment (Debye)
3.2938
4.9247
7.0780
Total heat capacity Cal/Mole-Kelvin
36.5180
41.4870
81.5680
Total Entropy Cal/Mole-Kelvin
97.6190
104.5540
151.3150
Total thermal Energy Kcal/Mol
99.2460
126.0270
227.1680
Table 11.
Comparison of thermochemical properties of MA, LPA and RPASMA.
The comparison of HOMO – LUMO energy details Figures 8 and 9 show the low energy gap in RPASMA formation from the donor – acceptor interactions of MA and LPA, which is resulting in the formation of less stable and highly reactive compound as shown in the Table 12 [19, 20].
Figure 8.
RPASMA HOMO PLOT.
Figure 9.
RPASMA LUMO PLOT.
Parameter
MA
LPA
RPASMA
HOMO (Orbital)
40
44
84
HOMO (Energy)
−0.27163
−0.25791
−0.25269
LUMO (Orbital)
41
45
85
LUMO (Energy)
−0.04288
−0.05366
−0.06529
Energy Gap
−0.22875
−0.20425
−0.1874
Table 12.
Comparison of HOMO – LUMO energies of MA, LPA and RPASMA.
The donor – acceptor interactions in the MA and LPA, give charge transfer mechanism in monoclinic RPASMA. The comparative study of the compounds show the parameters of solid state XRD measurements and the gaseous state DFT measurements are comparable. This study also confirms the product having inter and intra molecular hydrogen bonding, polarisation and van der Waals forces of attractions. The C2 space group results in noncenterosymmetric crystal structure. The FTIR vibrational study confirms the XRD and DFT parameters. The low energy gap of the RPASMA results in highly reactive nature and possibilities of compound in biological activity.
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Written By
Subramanian Usha and Charles Kanakam Christopher
Submitted: 25 January 2023Reviewed: 21 June 2023Published: 24 December 2023
Open access peer-reviewed chapter
