Structural and spectroscopic investigations of nonlinear optical crystal L-phenylalanine fumaric acid by DFT calculations

Article abstract of DOI:10.1016/j.molstruc.2019.03.013

The title compound L-phenylalanine fumaric acid has been synthesized in slow evaporation solution growth technique. The crystallinity of the crystal is confirmed by powder X-ray diffraction studies. Complete vibrational analysis and optimized molecular structure is obtained using density functional theory calculations. The vibrational contribution of each normal mode is figured out by calculating potential energy distribution with the aid of vibrational energy distribution analysis (VEDA 4) program. Experimental FT-infrared and FT-Raman spectra are recorded and analyzed. The second harmonic generation (SHG) efficiency of the grown crystal is measured by Kurtz – Perry technique. The calculated first hyperpolarizability and the highest occupied molecular orbital (HOMO) - lowest unoccupied molecular orbital (LUMO) energies confirm the nonlinear optical activity of the compound. The theoretical and experimental cutoff wavelength corresponding to electronic transitions are obtained. The most important hyper conjugative interactions leading to the stability and intramolecular charge transfer of the system is elucidated by natural bond orbital analysis. Information regarding the charge density distribution and chemical reactivity sites of the compound has been obtained by mapping molecular electrostatic potential.

Full text of DOI:10.1016/j.molstruc.2019.03.013

Accepted Manuscript
Structural and spectroscopic investigations of nonlinear optical crystal l-phenylalanine
fumaric acid by DFT calculations
P.V. Sreelaja, C. Ravikumar
PII:
S0022-2860(19)30265-0
DOI:
https://doi.org/10.1016/j.molstruc.2019.03.013
MOLSTR 26282
Reference:
To appear in: Journal of Molecular Structure
Received Date: 23 May 2018
Revised Date: 28 February 2019
Accepted Date: 5 March 2019
Please cite this article as: P.V. Sreelaja, C. Ravikumar, Structural and spectroscopic investigations of
nonlinear optical crystal l-phenylalanine fumaric acid by DFT calculations, Journal of Molecular Structure
(2019), doi: https://doi.org/10.1016/j.molstruc.2019.03.013.
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Graphical abstract

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Structural and spectroscopic investigations of nonlinear optical crystal L-
phenylalanine fumaric acid by DFT calculations
P. V. Sreelaja, C. Ravikumar*
Nanotechnology and Advanced Materials Research Centre, Department of Physics, CMS
College, Kottayam – 686 001, Kerala, India.
Abstract
The title compound L-phenylalanine fumaric acid has been synthesized in slow
evaporation solution growth technique. The crystallinity of the crystal is confirmed by powder
X-ray diffraction studies. Complete vibrational analysis and optimized molecular structure is
obtained using density functional theory calculations. The vibrational contribution of each
normal mode is figured out by calculating potential energy distribution with the aid of vibrational
energy distribution analysis (VEDA 4) program. Experimental FT-infrared and FT-Raman
spectra are recorded and analyzed. The second harmonic generation (SHG) efficiency of the
grown crystal is measured by Kurtz – Perry technique. The calculated first hyperpolarizability
and the highest occupied molecular orbital (HOMO) - lowest unoccupied molecular orbital
(LUMO) energies confirm the nonlinear optical activity of the compound. The theoretical and
experimental cutoff wavelength corresponding to electronic transitions are obtained. The most
important hyper conjugative interactions leading to the stability and intramolecular charge
transfer of the system is elucidated by natural bond orbital analysis. Information regarding the
charge density distribution and chemical reactivity sites of the compound has been obtained by
mapping molecular electrostatic potential.
Keywords: DFT; SHG; Charge transfer; NBO; Raman; MEP.
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*Corresponding Author: Dr. C. Ravikumar, Nanotechnology and Advanced Materials Research
Centre, Department of Physics, CMS College, Kottayam – 686 001, Kerala, India.
Telephone Number: +91 481 2566002, Fax Number: +91 481 2565002,
E-mail: rkr.ravi@gmail.com, ravikumar@cmscollege.ac.in
1. Introduction
Nonlinear optical (NLO) crystals having good stability, high transparency in the visible
region and superior nonlinear susceptibilities are noteworthy due to their influence on the
development of laser technology and optoelectronic industry. In recent years, amino acid
complexes having chiral symmetry along with zwitterionic nature due to the occurrence of donor
acceptor groups have been identified to have diverse application in science and industry [1-4].
Several L-phenylalanine crystals have been recognized as efficient NLO organic crystals due to
their high transparency in the range 380-800 nm and high damage threshold [5-11]. L-
phenylalanine is a naturally occurring aromatic, hydrophobic and α-amino acid produced by the
biological breakdown of protein in the body. It includes a phenyl ring having a side chain
substituted with an electron withdrawing -COOH moiety and electron donating NH2 moiety. The
occurrence of these donor-acceptor groups influences the optical proficiency of the crystals [12].
The wide transparency of organic acids makes them an excellent associate for amino acids to
fabricate highly efficient nonlinear optical materials [13,14].
Theoretical studies of structural dependence on NLO properties are analyzed and normal
modes of vibration of L-phenylalanine fumaric acid (PAF) are studied. First order
hyperpolarizability, molecular electrostatic potential and Mulliken charges obtained by density
functional theory level calculation are utilized to explore the molecular dynamics and chemical
behavior of PAF.
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2. Materials and Methods
2.1 Sample preparation
Transparent, colourless crystals of PAF has been prepared from the saturated aqueous
solution of L-phenylalanine and fumaric acid in 1:1 ratio as described by Alagar et al. [15]. The
resulting needle like crystal was repeatedly crystallized in double distilled water to obtain clear
crystals within 2 weeks.
2.2 Powder X-Ray diffraction measurement
The powdered sample of PAF crystal was subjected to Bruker D8 advanced powder X-ray
diffractometer (PXRD) with Cu Kα radiation of 40 kV, 30 mA. The specimen was scanned for
the angular range 20-80° 2 theta with the step size 0.0289°. The recorded spectrum is given in
Fig. 1. The PXRD data indicates that, PAF belongs to triclinic crystal system in P1 space group.
Lattice parameters of PAF are a=5.7144 Å, b=11.5489 Å, c=11.6149 Å and cell angles are
α=67.6971°, β=81.1306°, γ=79.3863°. Cell volume, V=694.048 Å3. All these parameters go in
agreement with the reported structural data and confirms the identity of the grown crystal [15].
The sharp nature of the peak indicates the purity and crystallinity of the crystal grown in the
laboratory.
2.3 Spectroscopic measurements
FT-infrared (FT-IR) spectrum of the grown sample is analyzed with compact Spectrum two
Lit A of Perkin Elmer. FT-IR spectra of the title compound in KBr pellet was recorded in the
range 4000 - 400 cm-1 at a resolution of 0.5 cm-1. The NIR-FT Raman spectrum of LPF crystal
was taken between 3500-50 cm-1 using Bruker RFS 27 stand alone FT-Raman spectrophotometer
which uses 1064 nm Nd:YAG laser source having 100 mW power. Ge-diode cooled with liquid
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nitrogen is used as detector. The spectral resolution is 2 cm-1. The UV-visible spectrum was
recorded in the range 200-500 cm-1 in water solvent, using Jasco V-650 high resolution UV-
visible spectrophotometer with double beam optics and photomultiplier tube detector. Resolution
of the measured spectrum is 1.0 nm.
2.4 Second harmonic generation analysis
The grown crystal of PAF was graded with a standard sieve to reduce the particle size (r) to
150<r<180 µm. Q switched high energy Nd:YAG laser having an incident wavelength of 1064
nm with a repetition rate of 10 Hz is used for the SHG measurement by powder reflection
technique [16]. 95% of the fundamental laser intensity was focused on to the sample and the
remaining 5% intensity was utilized as a reference to control fluctuations in incident beam. The
experimental second harmonic intensity deviation coming out from the sample as a function of
the fundamental intensity is plotted in Fig. 2. The PAF sample produced an output energy of 8.36
mJ which is 0.94 times that of potassium dihydrogen phosphate (KDP).
3. Computational techniques
All the quantum chemical computations performed in the present study is achieved using
Gaussian ’09 program package [17]. The hybrid exchange functional Becke three Lee-Yang-Parr
(B3LYP) is utilized along with the basis set 6-31G(d) for obtaining structural optimization and
harmonic frequency calculation. The normal mode frequencies obtained are real and establish
that the optimized structure corresponds to a true local minimum. The stereo electronic
interactions are interpreted by NBO 3.1 program incorporated in Gaussian '09 program at density
functional theory (DFT) level. The quantitative explanation of normal mode contributions of the
theoretical frequencies obtained from the potential energy distribution (PED) assignment is
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completed using vibrational energy distribution analysis program (VEDA 4) [18]. The group
contribution greater than or equal to 10% to each normal mode of vibration is interpreted after
repeated optimization. The theoretical IR and Raman spectra simulated by Gaussian calculations
have been plotted with full width at half maximum (FWHM) of 10 cm-1 using pure Lorentzian
band shapes. At the same time simulated Raman spectra is plotted with computed Raman
activities (Si) converted to relative Raman intensities (Ii) by means of Eq. (1) obtained from the
basic theory of Raman scattering [19, 20]. In Eq. (1) ν0 is the exciting frequency in cm-1, νi is the
vibrational wavenumber of the ith normal mode, h, c and k are universal constants, and ‘f’ is the
suitable scaling factor suitably commonly chosen for all the peak intensities.
f (ν o −ν
i )4 Si
Ii =

− hcν i





ν i 1− exp



kT



(1)
Experimental and simulated IR and Raman spectra are depicted in Fig. 4 and Fig. 5
respectively for visual comparison. The structural optimization is achieved by iteratively solving
the self-consistent field equation. This optimized molecular geometry produced 3N-6 positive
frequencies thus ensuring a minimum on the potential energy surface. The systematic errors of
calculated vibrational wavenumbers produced by incomplete basis set, ignoring anharmonicity
and electron correlation were corrected by a scaling factor 0.9613 [21-23].
The components of the hyperpolarizability tensor calculated by density functional theory
(DFT) method for the title compound are given in Table S1 (Supporting information). The
components of the first hyperpolarizability are calculated according to Thanthiriwatte and Silva
[24] by keeping the center of mass of the compound as the origin of the Cartesian coordinate
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system [24-25]. The first order hyperpolarizability of PAF is calculated as 2.79×10-30 esu which
is 4 times greater than the standard reference material KDP for which βtot is 6.85×10-31 esu [7].
4. Results and Discussion
4.1 Optimized geometry
The structural optimization of the PAF molecule was achieved using Gaussian ’09 program
package. The optimized geometrical parameters obtained by DFT method using B3LYP
functional are tabulated in Table 1 and the optimized structure is given in Fig. 3. The magnitude
of bond angle O22-C21-O23 has enhanced by 5.83° compared to the experimental data [15] due to
the intramolecular hydrogen bonding of O23 atom with H24 atom of aliphatic fumaric acid. The
lowering of bond angle C15-C21-O23 (112.15°) compared to that of C15-C21-O22 (117.45°), and
elongated bond length of C21-O23 compared to that of C21-O22 is due to the intramolecular
hydrogen bond formed between phenylalanine and the dicarboxylic acid. The presence of
…
intramolecular hydrogen bonding is affirmed by the decrease in O23 H24 distance and increase in
bond length of O25-H24 as well as C21-O23. The increase in O22-C21-O23 bond angle also supports
the above inference. Likewise, there exists strong N-H…O interaction between N17 and O27 which
…
is reflected in the lengthening of C26-O27, N17-H20 bonds and short distance between H20 O27.
Lengthening of these bonds is due to the hyperconjugative interaction between lone pair orbital
LP1 (O27) and σ∗(N17-H20) [26]. The occurrence of these two hydrogen bonded interactions gives
increased stability for the system.
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•
•
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Position for Figure 2
Position for Figure 3
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•
Position for Table 1
4.2 Natural Bond Orbital and “bond bending” Analysis
For the analysis of electronic wavefunction, natural bond orbital (NBO) method is used by
NBO 3.1 program [21]. The NBO analysis incorporates a collection of mathematical procedures
to describe wavefunction in terms of Lewis to non-Lewis form of contributions. The most
significant exchanges among donor Lewis type NBOs and acceptor non-Lewis NBOs are
reported in Table 2. The quantitative contribution of s and p-character, occupancy of natural
bonds and lone pairs, are calculated and are given in Table 2. Stabilization energy ꢀEij is
evaluated using second order perturbation theory as proposed by Reed et.al. [22].
Stability of the donor-acceptor interaction is observed by the amount of electron density
delocalization among the bonding and antibonding NBOs. The strong intramolecular O-H...O
hydrogen bonding and charge delocalization is reflected in the stabilization energy E2 of LP
(O23) to antibonding H24-O25. The electron density (ED) value (0.03149 e) of the antibonding
σ∗(H24-O25) is a measure of intramolecular charge delocalization and hence the weakening of the
bond marked by the elongation of bond length H24-O25. The stabilization of the system is
achieved by hyperconjugative orbital overlap interaction between π(C-C) and π∗(C–C) of the
phenyl ring.
The hyperconjugative interaction between lone pair LP1 (O27) with σ∗(N17-H20) is of much
importance because of the decreased electron density of non-Lewis orbital, and its effect on bond
length and molecular stability. The elongation of N17-H20 bond length is due to the increased p-
character of σ(N17-H20) ie. 2.72 %. When the percentage p contribution of LP1(O27) is 42.79%
while that of LP2(O27) is 99.62 %. ie. LP2(O27) is comparable to a pure π type lone pair orbital
participating in electron donation to the σ∗(N17-H20) orbital for the interaction LP2(O27)→ σ∗(N17-
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H20). The low ED value of σ∗ (N-H) can be due to the electron density redistribution effect which
is significant for N-H…O bonds [26]. The electron density 0.03039 e of σ∗(N17-H20) leads to a
moderate stabilization energy 15.12 kJmol-1 while E2 of σ∗(H24-O25) for LP1(O23)→ σ∗(H24-O25)
is 38.514 kJmol-1. The variances in E2 energies are due to accumulation of electron density not
only from the n(O) of hydrogen-acceptor but also from the whole molecule. The E2 values of
donor acceptor interaction among LP (O23) and LP (O27) with σ∗(H24-O25) and σ∗(N17-H20)
respectively enumerate the strength of intramolecular hydrogen bonding. Interactions between
LP2 (O25)→ π∗(C26–O27) gives out an enormous energy due to the accumulation of charge density
and intensive interaction between N-H leading to strong delocalization.
The natural hybrid orbital (NHO) directional analysis in the NBO output exhibits the angular
variation of NHO with the direction of line joining the two nuclear centres giving an insight
about deviation (DEV) and thereby charges transfer. It also presents evidence about the direction
of geometry changes followed due to optimization of the molecular structure. For NHOs in σ
bonding orientation, DEV values are ∼ 0°, whereas perpendicular π type orientation corresponds
to DEV = ∼90° [23]. The DEV tabulated in Table 4 gives bending angles of most significant
bonds. The existence of deviation for all the ring atoms represents the conjugative effect and
increase of strains at the active centre of the compound [27]. Among the ring carbon atoms, C4
shows highest DEV due to substitution effect [28-29]. Similarly, a remarkable deviation of 2.9°
is obtained for C15 atom participating σ (C12–C15) due to the presence of zwitterionic groups
+
NH3 and COO-. The extent of resonance interaction is high when O23 is in the same plane with
. . .
fumaric acid moiety. Due to strong O25-H24 O23 intramolecular hydrogen bonding interaction of
stabilization energy 38.514 kJmol-1, O23 is twisted away from the line of center of σ (C21–O23) by
angle 2.9°. O25 and C26 shows a markable deviation of 5.1° and 4.0° from the line of center of
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bonds σ (H24–O25) and σ (O25–C26) respectively whereas the NHO of H24 goes in line with σ
(H24–O25) axis and DEV value comes under the threshold printing limit of 1.0°. The alignment of
H24 atom is not deviated from σ (H24-O25) bond axis since it lies in the strong charge transfer
path.
•
•
•
Position for Table 2
Position for Table 3
Position for Table 4
4.3 Vibrational spectral analysis
The vibrational spectral analysis is completed on the basis of the characteristic vibrations
of the substituted pyridine. The experimental FT-IR, FT- Raman spectra and B3LYP/6-31G(d)
level simulated theoretical spectra are shown in Figs. 4 and 5. The detailed analyses of
vibrational wavenumbers and the confirmation of the presence of various functional groups
(Table 5) are discussed below. The wavenumber discrepancies of simulated and experimental
spectra are owing to the dissimilar circumstances in which they are obtained. Theoretical
spectrum is achieved from an isolated optimized molecule in gas phase and experimental spectra
is measured in solid phase, which may get influenced by inter and intramolecular interactions.
•
Position for Table 5
Position for Figure 4
Position for Figure 5
•
•
4.3.1 Phenyl ring vibrations
Normal mode vibrations of monosubstituted benzene ring is studied and assigned with
Wilson’s numbering system [30]. Monosubstituted benzene has five C-H stretching modes 2, 7a,
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7b, 20a, and 20b out of which only 20a, 20b, 2 and mode 7a are active in the IR spectrum. A
very strong band at 3061 cm-1 in Raman spectra is assigned as mode 20a while a strong peak at
3029 cm-1 in the IR spectra corresponds to mode 20b. The band found at 3041 cm-1 in Raman is
identified as mode 2. The strong band in IR at 3000 cm-1 and medium band at 3008 cm-1 in
Raman is assigned to mode 7a of the phenyl ring. The intensity of the C-H ring stretching is
+
reduced and multiple weak bands are observed in IR around 3029 cm-1 because NH3 and CH2
group stretching vibrations fall in the same region [31]. Two doubly degenerate C-C stretching
of the benzene ring is attributed to mode e2g and e1u. The mode 8a appears as strong peaks at
1591 cm-1 and 1601 cm-1 in IR and Raman respectively, while 8b is found at a lower
wavenumber 1586 cm-1 as a medium intensity peak in Raman. Mode 19a is assigned to weak
band at 1491cm-1 in Raman and a strong peak at 1490 cm-1 in IR whereas mode 19b is identified
at 1446 cm-1 in IR as weak band and at 1445 cm-1 in Raman spectra as medium peak. Normal
mode 14 is found as a medium band at 1304 cm-1 in IR and a weak band at 1312 cm-1 in Raman.
Among the C-C aromatic stretching modes 8a, 19a and 14 are simultaneously active in both IR
and Raman measurements which is an indication of charge transfer interaction. Modes 8a and
19a are coupled with C-H bending motion of the phenyl ring to a greater extend which is clearly
depicted in Table 5.
The C-H in plane bending vibrations are assigned to modes 3, 9a, 15, 18a and 18b. The bands
at 1339 cm-1 and 1341 cm-1 in both IR and Raman respectively correspond to mode 3. The weak
band at 1165 cm-1 in IR corresponds to mode 9a and a medium band at 1160 cm-1 in Raman
denotes mode 15. The medium peak at 1075 in IR refers to mode 18b. Radial skeletal vibrations
of monosubstituted benzene is denoted by 1, 12, 6a and 6b in Wilson’s numbering system [30].
The frequency of 6a is highly variable from 300 cm-1 to 530 cm-1 while that of 6b is identified at
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a rather stable range 605-630 cm-1 [30]. The weak bands found in the Raman spectra at 315 cm-1
and 620 cm-1 are identified as 6a and 6b modes respectively. A strong polarized peak around
1000 cm-1 is expected in Raman for mode 12 [30]. The strong band appearing in Raman at 1002
cm-1 is assigned for mode 12. Other radial deformations, out of plane CH bending and skeletal
vibrations are indicated in Table 5.
4.3.2 CH2 Vibrations
The CH2 antisymmetric stretching is expected around 2926 cm-1 in IR and Raman [32]. The
strong band observed at 2949 cm-1 in IR and a strong band at 2948 cm-1 in Raman is attributed to
asymmetric CH2 stretching. The shifting of stretching vibration to a longer wavenumber side is
due to the strain on the phenyl ring. The CH2 wagging mode is at 1270 cm-1 and 1275 cm-1 in IR
and Raman respectively which is well inside the range of 1400–900 cm-1 [32, 33].
4.3.3 Carbonyl vibrations
Amino acids in the zwitterionic form having deprotonated carboxyl group COO- gives two
significant characteristic bands, due to asymmetric and symmetric stretching. Asymmetric
stretching falls in the region 1605-1585 cm-1 whereas symmetric stretching is expected within
1425-1393 cm-1 [34-35]. The medium IR band present at 1695 cm-1 and a strong Raman
equivalent at 1690 cm-1 is assigned for asymmetric stretching of COO-. The blueshifting is due to
the intramolecular O-H…O hydrogen bonding, thereby increasing the force constant of the bond.
Symmetric COO- stretching is observed as medium bands in Raman at 1210 cm-1. The
wavenumbers corresponding to this mode is found to be significantly lowered owing to the
decrease in double bond character of CO group. The lowering of the COO- symmetric stretching
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…
wavenumber is an indication of hydrogen bond formed in O25-H24 O23 since hydrogen bonding
to COO- can decrease the wavenumber upto 60 cm-1 [35].
+
4.3.4 NH3 Vibrations
N-H stretching falls in the region 3500 to 3200 cm-1 [34]. The position of absorption is
sensitive to the degree of hydrogen bonding and protonation results in broadening and
weakening of the peaks. The expected range of asymmetric stretching after protonation is amid
+
3300-3100 cm-1 [34,36]. The broad very strong band at 3079 cm-1 in IR is assigned to NH3
asymmetric stretching. Lowering of asymmetric stretching wavenumber is due to N-H…O
+
hydrogen bond formation. For amino acids with NH3 group, the symmetric stretching come in
between 3100-2800 cm-1 [35]. The strong Raman band formed at 2977 cm-1 is assigned for
+
symmetric stretching of NH3 which confirms the formation of zwitterionic state [37,38].
+
The strong band at 1612 cm-1 in IR corresponds to NH3 asymmetric deformation which comes
in its normal range 1615-1580 cm-1 [7]. A medium band expected within the range 1550-1500
cm-1 for symmetric deformation is found slightly blueshifted as a strong band at 1567 cm-1 in the
+
IR spectrum. The rocking mode of NH3 is heavily coupled with stretching and bending
vibrations of C26-O25 which is mentioned in Table 5. Simultaneous presence of a medium band in
IR and a very weak band in Raman at 481cm-1 is due to C21-C15-N17.
4.3.5 C-C, C-H, and C-N vibrations
C-H stretching vibrations in Raman spectrum is expected near 3000 cm-1 [32]. The strong band
at 2926 cm-1 in Raman is identified as C-H stretching vibration. The characteristic C-N
+
stretching vibration of the C-NH3 is found around 1000 cm-1 [39]. This range comes well with
in stretching vibrations of C-C [39]. The medium intensity band present at 994 cm-1 in IR is
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assigned for C-N stretching coupled with C-C stretching. The CCH bending of fumaric acid
moiety is identified as strong band in IR at 1256 cm-1 while that of phenylalanine moiety is found
at 1210 cm-1 as a medium band in Raman due to its coupling with COO- stretching vibration. CH
deformation of zwitterionic amino acid falls in the range 1340-1315 cm-1 [32]. The weak peak at
1370 cm-1 in IR and a medium peak at 1368 cm-1 in Raman is due to deformation involving C15
and H16.
4.3.6 Carboxylic acid vibrations
Isomeric unsaturated dicarboxylic acids exhibit very broad band in the region 3400-3600 cm-1
for O-H stretching vibration [32]. The broad band in IR at 3416 cm-1 corresponds to O-H
stretching of carboxylic group in the fumaric acid moeity. The normal range of C=O in
carboxylic acid is 1725-1700 cm-1, while intramolecularly hydrogen bonded acids lower the
frequency range to 1680-1650 cm-1 [32]. The strong peaks at 1712 cm-1 in IR and at 1706 cm-1 in
Raman corresponds to C32=O33 stretching which agrees well with the normal range. The
stretching of C26=O27 gives a downshifted strong band in IR at 1631 cm-1 and medium band in
+
Raman at 1641 cm-1. The redshifting is due to its strong coupling with NH3 bending vibrations
…
indicating the formation of strong intramolecular hydrogen bond between N17-H20 O27. The
hyperconjugative interaction between LP(1) O27 and LP(2) O27 with σ∗(N17-H20) resulting in a
high stabilization energy value of 15.12 and 20.41 kJmol-1 respectively shows the presence of
intramolecular charge delocalization and thereby hydrogen bond formation. The strong peak at
1408 cm-1 in IR and a weak peak at 1411 cm-1 in Raman is due to interacting C-O stretch and C-
O-H in plane deformation vibration of carboxylic acid group.
4.3.7 Hydrogen bonding
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The formation of hydrogen bonds in phenyl alanine compounds is important because bond
+
formation stabilizes the protein structures through dipole interactions [40]. The presence of NH3
and COO- makes it a zwitterion stabilized with O-H…O and N-H…O intramolecular hydrogen
bonds. The bond distance between O25 and O23 is 2.6 Å which comes in the range of strong
hydrogen bonds (2.4 – 2.7 Å) [41]. The band at 120 cm-1 in Raman is assigned to the torsional
…
vibration of C21-O23 H24-O25 and a very strong peak observed at 72 cm-1 is assigned for bending
…
…
+
of O25-H24 O23. The stretching of O23 H24 is found to be coupled with NH3 torsional vibration.
An overview on the vibrational band assignments point out that blueshift of stretching
wavenumbers and redshift of deformation modes from the free ion values are the consequences
of strong O-H…O hydrogen bonding.
4.4 Absorption spectra and solvent effects
The electronic absorption wavelengths of PAF computed in the gas phase and in aqueous
solvent are presented in Table 6. The solvent effect was calculated using polarizable continuum
model (PCM) method at TD CAM-B3LYP/6-31G(d) level. The experimental UV-vis spectra
using aqueous solvent and simulated spectra in gas phase as well as water are shown in Fig. 6.
•
•
Position for Figure 6
Position for Figure 7
Position for Table 6
•
The electron donating highest occupied molecular orbital (HOMO) is concentrated over the
substituted phenyl ring of amino acid moiety and lowest unoccupied molecular orbital (LUMO)
of fumaric acid moiety receives the electron. The pictorial representation of HOMO-LUMO is
presented in Fig. 7. Thus, a charge transfer interaction takes place within the compound from
molecular orbitals 74 to 75 with ‘A’ symmetry. The electron transference from HOMO to
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LUMO is depended on the energy difference between them which has significant role in electro-
optic properties, reactivity and absorption in UV-visible spectra. The energy gap calculation at
B3LYP/6-31G(d) level in gas phase is calculated as 3.793 eV, which is much smaller than most
of the phenylalanine and fumaric acid complexes (5.405 eV for L-phenylalanine-benzoic acid,
5.983 for 4-Chloro-DL-phenylalanine, 5.532 eV for L-valinium fumarate, 5.932eV for N-acetyl-
L-phenylalanine [8,38,42,43]. The formation of strong O-H…O and N-H…O hydrogen bond amid
the charged moieties reduced the energy gap substantially with the formation of charge transfer
axis [7].
The experimental absorption spectrum in water gives a strong transition at 209 nm owing to
the π → π* transition in the aromatic ring. The simulated spectrum with water solvent shows a
strong transition at 234 nm with oscillator strength of 0.0102. A high transmittance percentage in
the entire visible region (380 nm – 800 nm) makes PAF a promising candidate for nonlinear
optical applications.
The conceptual DFT is a valuable tool to obtain quantitative information about the chemical
reactivity of compounds [44, 45]. In the conceptual DFT, electronic density is the fundamental
tool to describe the electronic states of atoms and molecules. A series of chemical concepts such
as electronegativity, hardness is quantitatively studied by using a set of chemical reactivity
indices derived from HOMO and LUMO energy [45, 46]. Chemical reactivity indices such as
chemical hardness (η), chemical potential (µ), softness (S) and electrophilicity index (ω) were
calculated by the following equations [47-52].
η = (I - H) / 2
µ = -(I + H) / 2
S = 1/(η)
(2)
(3)
(4)
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ω = µ2/2η
(5)
where I and H are ionization potential (I = -EHOMO) and electron affinity (H = -ELUMO) of a
molecular system. Electron affinity denotes the capacity to receive just one electron from a
donor. Nevertheless, in several types of bonding like covalent hydrogen bonding, fractional
charge transmission occurs within the system. Calculated value of global reactivity coefficients
of PAF are presented in Table 7. The negative value of chemical potential (-7.73 eV) indicates
that the compound is stable while the low magnitude of chemical hardness (1.90 eV) suggests
charge transfer interaction [53, 54].
4.5 Mulliken atomic charge and molecular electrostatic potential (MEP) Analysis
Dipole moment, polarizability and electronic structure of a molecular system are influenced
by the atomic charge distribution. The Mulliken atomic charges calculated at B3LYP/6-31G(d)
level using Gaussian ’09 is shown in Fig. 8. Among the hydrogen atoms and oxygen atoms
present in PAF, H24 and O23, which are involved in hydrogen bonding interaction possess the
highest positive (0.470 e) and negative (-0.651 e) atomic charge respectively. So, extensive
charge delocalization is suggested by the negative value of O23 and positive atomic charge of
H24. Atomic charges corresponding to all phenyl ring carbon atoms are negative except for C4,
pointing to the fact that electronic delocalization effect is additionally strong among carbon
atoms leading to charge transfer between substituents. C21 atom which is attached to the electron
withdrawing O23 atom exhibits the highest positive charge in order to pull out the partial charges
from the compound.
•
Position for Figure 8
16

ACCEPTED MANUSCRIPT
MEP mapping allows visual comparison of electrostatic potentials of each atom in a
molecular system by providing different colors like red, orange, green, blue in the decreasing
order of potential. MEP surface analysis allows recognizing the reactive sites for electrophilic as
well as nucleophilic attack in hydrogen bonded interactions. The MEP surface plotted by
mapping the electrostatic potential onto the isosurface range of -0.08847 to +0.08847 showing
the relative polarity of LPF molecule obtained at B3LYP/6-31G(d) level is given in Fig. 9. The
MEP map suggests electron density is mainly concentrated over lone pair of electronegative
oxygen atom making it the most reactive part of the molecule while highest positive potential is
+
found around nucleophilic H atoms in NH3 group. These electrophilic and nucleophilic sites
thus give a pictorial representation of regions from where the compound can have intra
molecular interactions and confirm the existence of N-H…O bond formation.
•
Position for Figure 9
5. Conclusion
The experimentally synthesized PAF crystals were analyzed for elaborate vibrational spectral
investigation by FT-IR, FT-Raman and UV-visible studies. Molecular structure optimization and
harmonic vibrational frequencies are obtained using density functional theory at B3LYP/6-
31G(d) level. Existence of O-H…O and N-H…O hydrogen bond leading to the charge transfer
interaction responsible for the nonlinear optical activity is studied by NBO analysis. Down shift
+
+
of NH3 stretching wavenumber and upshift of NH3 deformation indicates the presence of N-
H…O hydrogen bonding. The appearance of normal mode 8a, 19a and 14 of the phenyl ring
simultaneously in FT-IR and FT-Raman spectra is an evidence for charge transfer between
phenylalanine and the dicarboxylic acid moieties. Mapping of molecular electrostatic potential
+
suggests oxygen in COO- group as strong electrophilic site and hydrogen in NH3 as nucleophilic
17

ACCEPTED MANUSCRIPT
site from where PAF can form intramolecular hydrogen bond. The low HOMO-LUMO energy
gap and transparency to the entire visible region suggests PAF a potential candidate for NLO
application.
18

ACCEPTED MANUSCRIPT
References
[1] M. P. Mohamed, P. Jayaprakash, M. Nageshwari, C. R. Kumari, P. Sangeetha, S. Sudha, G.
Mani, M. L. Caroline; J. Mol. Struct.; 1141 (2017) 551-562.
[2] M. Shkir; J. King Saud University-Science; 29 (2017) 70-83.
[3] G. R. Dillip, C.M. Reddy, B.D.P. Raju; J. Minerals and Mater. Charact. and Eng.; 10
(2011) 1103-1110.
[4] A. B. Ahmed, H. Feki, Y. Abid, C. Minot; Spectrochim. Acta Part A: Mol. and Biomol.
Spectroscopy; 75 (2010) 1315-1320.
[5] A. C. Peter, M. Vimalan, P. Sagayaraj, J. Madhavan; Physica B; 405 (2010) 65-71.
[6] A. C. Peter, M. Vimalan, P. Sagayaraj, T. Rajesh Kumar, J. Madhavan; Int. J. ChemTech
Research; 2, (2010) 1445-1453.
[7] N. Elleuch, A. B. Ahmed, H. Feki, Y. Abid, C. Minot; Spectrochim. Acta Part A: Mol. and
Biomol. Spectroscopy; 121 (2014) 129-138.
[8] K. Govindarasu, E. Kavitha; Spectrochim. Acta Part A: Mol. and Biomol. Spectroscopy; 133
(2014) 799-810.
[9] F. Yogam, I. V. Potheher, A. C. Peter, S. Tamilselvan, A. L. Rajesh, M. Vimalan, P.
Sagayaraj; Advances in Applied Science Research; 2 (2011) 261-268.
[10] M. Prakash, M. L. Caroline, D. Geetha; Spectrochim. Acta Part A: Mol. and Biomol.
Spectroscopy; 108 (2013) 32-37.
[11] T. Sujatha, A. C. Peter, M. Vimalan, J. Merline Shyla, J. Madhavan; Physica B; 405 (2010)
3365-3370.
[12] M. Blanchard-Desce, C. Runser, A. Fort, M. Barzoukas, J. M. Lehn, V. Bloy, V. Alain;
Chem. Phys.; 199 (1995) 253-261.
19

ACCEPTED MANUSCRIPT
[13] M. N. Bhat, S. M. Dharmaprakash; J. Crystal Growth; 236 (2002) 376-380.
[14] J. F. Nicoud, R. J. Twieg, D. S. Chemla, J. Zyas; Opt. Prop. Org. Molecules and Crystals;
Academic Press, London, (1987).
[15] M. Alagar, R. V. Krishnakumar, K. Rajagopal, M. Subha, Nandhini, S. Natarajan; Acta
Cryst.; E59 (2003) o952 - o954.
[16] S. K. Kurtz, T. T. Perry; J. Appl. Phys.; 39 (1968) 3798-3813.
[17] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman,
G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara,
K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai,
T. Vreven, J.A. Montgomery Jr., J. E Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E.
Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A.
Rendell, J.C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K.
Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A.
D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox; Gaussian 09,
Revision A.02; Gaussian, Inc. Wallingford CT, (2009).
[18] M. H. Jamroz; Vibrational energy distribution analysis VEDA 4; Warsaw, (2004).
[19] P. L. Polavarapu; J. Phys. Chem.; 94 (1990) 8106-8112.
[20] G. Keresztury, J. M. Chalmers, P. R. Griffith (Eds.); Raman Spectroscopy: Theory in
Handbook of Vibrational Spectroscopy; vol. 1, John Wiley & Sons Ltd., New York, (2002).
20

ACCEPTED MANUSCRIPT
[21] E. D. Glendening, A. E. Reed, J. E. Carpenter, F. Weinhold; NBO Version 3.1; TCI,
University of Wisconsin, Madison, (1998).
[22] A. E. Reed, L. A. Curtiss, F. Weinhold; Chem. Rev.; 88 (1988) 899-926.
[23] E. D. Glendening, C. R. Landis, F. Weinhold; WIREs Comput. Mol. Sci.; 2 (2012) 1-42.
[24] K. S. Thanthiriwatte, K. M. Nalin de Silva; J. Mol. Struct.; 617 (2002) 169-175.
[25] M. Adant, M. Dupuis, J. L. Bredas; Int. J. Quan. Chem.; 56 (1995) 497-507.
[26] Y. Yang, W. Zhang, X. Gao; Int. J. Quan. Chem.; 106 (2006) 1199-1207.
[27] K. Anitha, V. Balachandran, B. Narayanan; Int. J. Curr. Res. and Aca. Rev.; 3 (2015) 70-
89.
[28] D. Arul Dhas, I. Hubert Joe, S. D. D. Roy, S. Balachandran; Spectrochim. Acta Part A:
Mol. and Biomol. Spectroscopy; 79 (2011) 993-1003.
[29] B. Edwin, M. Amalanathan, I. Hubert Joe; Spectrochim. Acta Part A: Mol. and Biomol.
Spectroscopy; 96 (2012) 10-17.
[30] G. Varsanyi; Vibrational Spectra of Benzene Derivatives; Academic Press, New York,
(1969).
[31] B. J. M. Rajkumar, V. Ramakrishnan; Spectrochim. Acta Part A: Mol. and Biomol.
Spectroscopy; 58 (2002) 1923-1934.
[32] D. Lin-Vien, N. B. Colthup, W. G. Fateley, J. G. Graselli; The Hand Book of Infrared and
Raman Characteristic Frequencies of Organic Molecules; Academic Press, New York,
(1991).
[33] J. G. Mesu, T. Visser, F. Soulimani, B. M. Weckhuysen; Vib. Spectrosc.; 39 (2005) 114-
125.
21

ACCEPTED MANUSCRIPT
[34] L. J. Bellamy; The Infrared Spectra of Complex Molecules; Chapman and Hall, London,
(1980).
[35] G. Socrates; Infrared characteristic group frequencies; John Wiley & Sons; New York,
(1980)
[36] J. Mohan; Organic Spectroscopy—Principles and Applications; second ed., Narosa
Publishing House, New Delhi, (2001).
[37] M. Alagar, R. V. Krishnakumar, K. Rajagopal, M. Subha Nandhini, S. Natarajan; Acta
Cryst.; E59 (2003) o952-o954
[38] X. Sun, X. Liu, X. Xu, P. Sun; Optik – Int. J. for Light and Electron Optics; 127 (2016)
4881-4888
[39] F. R. Dollish, W. G. Fateley, F. F. Bentley; Characteristic Raman Frequencies of Organic
Compounds; Wiley, New York, (1973).
[40] M. Amalanathan, I. Hubert Joe, V. K. Rastogi; J. Mol. Struct.; 1006 (2011) 513-526.
[41] I. D. Brown; Acta Cryst.; A 32 (1976) 24-31.
[42] X. Liu, X. Xu, C. Zhang; Spectrochim. Acta Part A: Mol. and Biomol. Spectroscopy; 136
(2014) 162-167.
[43] B. Raja, V. Balachandran, B. Revathi; Spectrochim. Acta Part A: Mol. and Biomol.
Spectroscopy; 138 (2015) 283-295.
[44] P. W. Ayers, R. G. Parr; J. Am. Chem. Soc.; 122 (2000) 2010-2018.
[45] A. Poater, A. G. Saliner, M. Sola, L. Cavallo, A. P Worth; Expert Opin. Drug Deliv.; 7
(2010) 295-305.
[46] A. Poater, A. G. Saliner, R. Carbo-Dorca, J. Poater, M. Sola, L. Cavallo, A. P. Worth; J.
Comp. Chem.; 30 (2009) 275-284.
22

ACCEPTED MANUSCRIPT
[47] R. G. Parr, R. A. Donnelly, M. Levy, W. E. Palke; J. Chem. Phys.; 68 (1978) 3801-3807.
[48] R. G. Parr, L. Szentpaly, S. Liu; J. Am. Chem. Soc.; 121 (1999) 1922-1924.
[49] R. G. Pearson; J. Am. Chem. Soc.; 85 (1963) 3533-3539.
[50] R. G. Parr, R. G. Pearson; J. Am. Chem. Soc.; 105 (1983) 7512-7516.
[51] P. K. Chattaraj, H. Lee, R. G. Parr; J. Am. Chem. Soc.; 113 (1991) 1855-1856.
[52] P. K. Chattaraj, P. V. R. Schleyer; J. Am. Chem. Soc.; 116 (1994) 1067-1069.
[53] P. K. Chattaraj, S. Giri; J. Phys. Chem. A; 111 (2007) 11116-11121.
[54] S. Chitrambalam, D. Manimaran, I. H. Joe, V. K. Rastogi, I. U. Hassan; Opt. Mater.; 75
(2018) 285-296.
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Figure captions
Fig. 1 Powder X-ray diffraction pattern of PAF
Fig. 2 Variation of SHG in PAF for the particle size 150<r<180 µm as compared to KDP
Fig. 3 Optimized structure of PAF calculated at B3LYP/6-31G(d)
Fig. 4 (a). The FT-IR spectrum of PAF molecule in the wavenumber range 4000-400 cm-1
(b). The simulated infrared spectrum of PAF molecule computed at B3LYP/6-31G(d) basis set.
Fig. 5 (a). The FT-Raman spectrum of PAF molecule in the wavenumber range 4000-50 cm-1.
(b). The simulated Raman spectrum of PAF molecule computed at B3LYP/6-31G(d) basis set.
Fig. 6 (a) Experimental UV-Vis spectrum of PAF molecule in water solvent
(b) Simulated UV-vis spectrum in water for PAF molecule calculated with the PCM CAM-
B3LYP/6-31G(d) method
(c) Simulated UV-vis spectrum of PAF molecule in the gas phase
Fig. 7 (a) HOMO plot of PAF at B3LYP/6-31G(d) (b) LUMO plot of PAF at B3LYP/6-31G(d)
Fig. 8 Mulliken charge distribution chart of PAF molecule
Fig. 9 Molecular electrostatic potential map of PAF molecule calculated at B3LYP/6-31G(d)
24

ACCEPTED MANUSCRIPT
Table 1. Optimized geometrical parameters of PAF by B3LYP /6-31G(d) in comparison with
XRD data.
Bond length (Å)
Bond angle (°)
Dihedral angle (°)
Parameter Cal Expa
Parameter
Cal
Expa
Parameter
Cal
Expa
-1.13
0.83
0.27
-179.19
0.29
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
C1-H7
C2-H8
C3-H9
C1-C2-C3
C2-C3-C4
C3-C4-C5
C4-C5-C6
C6-C1-H7
C1-C2-H8
C2-C3-H9
C4-C5-H10
C5-C6-H11
C5-C4-C12
C4-C12-H13
120.23
121.07
117.91
120.84
120.44
119.89
119.45
119.52
119.60
120.86
108.56
C1-C2-C3-C4
C2-C3-C4-C5
C3-C4-C5-C6
C5-C6-C1-H7
H7-C1-C2-H8
C1-C2-C3-H9
C3-C4-C5-H10
C4-C5-C6-H11
C2-C3-C4-C12
C5-C4-C12-H13
C3-C4-C12-H14
C3-C4-C12-C15
C4-C12-C15-H16
C4-C12-C15-N17
1.39 1.39
1.40 1.38
1.40 1.39
1.40 1.38
1.39 1.38
1.09 0.93
1.09 0.93
1.09 0.93
1.09 0.93
1.09 0.93
1.52 1.51
119.91
120.97
118.66
120.39
120.22
120.22
119.45
119.16
119.53
121.01
109.48
-0.15
0.26
-0.16
179.79
-0.12
178.74
-179.97
179.61
-179.68
159.27
-138.6
103.15
-170
178.90
-179.77
178.89
-179.29
141.40
-155.04
83.23
172.19
-69.39
-56.04
-176.04
64.01
54.30
-46.70
17.80
89.31
-105.16
10.47
C5-H10
C6-H11
C4-C12
C12-H13
C12-H14
C12-C15
C15-H16
C15-N17
H18-N17
H19-N17
H20-N17
C15-C21
C21-O22
C21-O23
1.10 0.97 C4-C12-H14
1.09 0.97 C4-C12-C15 115.37 114.91
110.4 108.52
C12-C15-H16
C12-C15-N17
C15-N17-H18
C15-N17-H19
C15-N17-H20
C12-C15-C21
C15-C21-O22
107.83
1.53 1.54
1.10 0.98
1.51 1.49
1.02 0.89
1.05 0.89
1.04 0.89
1.58 1.52
1.23 1.25
1.29 1.25
1.58 1.83
1.02 0.82
1.32 1.32
1.23 1.21
1.49 1.48
1.08 0.93
1.34 1.32
1.09 0.93
1.48 1.49
1.21 1.20
1.36 1.31
0.98 0.82
1.81 2.05
1.84 2.18
109.36
113.78
115.47
101.55
111.89
115.24
117.45
112.15
115.62
-51.65
-74.75
161.2
111.96 C12-C15-N17-H18
109.47 C12-C15-N17-H19
109.48 C12-C15-N17-H20
53.64
109.47
C4-C12-C15-C21
68.58
111.08 H16-C15-C21-O22
-84.97
-18.35
105.09
-91.12
15.08
117.97 N17-C15-C21-O23
...
C15-C21-O23
115.22 C15-C21-O23
H
24
...
...
C21-O23
H
24
129.89 C21-O23 H24-O25
…
...
O23 H24
O22-C21-O23 130.39 124.56 O23 H24-O25-C26
...
H24-O25
O25-C26
C26-O27
C26-C28
C28-H29
C28-C30
C30-H31
C30-C32
C32-O33
O34-C32
H35-O34
O23 H24-O25
H24-O25-C26
O25-C26-O27
O25-C26-C28
C26-C28-H29
C26-C28-C30
C28-C30-H31
C28-C30-C32
C30-C32-O33
C30-C32-O34
C32-O34-H35
168.16 H24-O25-C26-O27
109.46 H24-O25-C26-C28
123.65 O27-C26-C28-H29
112.71 O25-C26-C28-C30
119.92 C26-C28-C30-H31
120.09 C26-C28-C30-C32
118.36 C28-C30-C32-O33
123.30 C28-C30-C32-O34
121.66 C30-C32-O34-H35
113.05
5.75
175.76
112.55
126.23
111.81
117.08
120.89
120.41
124.53
123.69
113.6
-1.17
179.1
-174.76
-172.75
-172.24
2.29
-177.70
169.16
-11.15
168.16
-177.17
-176.93
0.06
179.57
-179.05
0.92
-179.91
109.44
105.92
…
H20 O27
…
O23 H19
aTaken from Ref [15]

ACCEPTED MANUSCRIPT
Table 2. NBO results showing the formation of Lewis and Non-Lewis orbitals by the valence
hybrids corresponding to the intramolecular N-H…O hydrogen bonds of PAF
BOND (A-
B)
Energy
EDA(
%)
ED (e)
0.03149
0.05995
0.08052
EDB(%)
24.67
33.67
67.46
66.33
NBO
S (%) P (%)
100.00
(kJmol-1)
0.8679 (s)H
-
σ∗(H24-O25)
σ∗(C21-O23)
σ∗(O25-C26)
σ(C21-O23)
2148.302
1605.266
1050.270
75.33
66.33
32.54
33.67
-0.4967 (sp1.95
)
33.84 66.08
32.86 67.02
40.87 58.83
32.93 67.00
29.01 70.75
32.86 67.02
40.87 58.83
58.98 40.99
33.07 66.83
57.16 42.79
O
0.8144 (sp2.04
-0.5803(sp 1.44
)
C
)
O
0.5705(sp2.03
-0.8213(sp2.44
)
O
)
C
0.5803(sp 2.04
)
C
1.99683 -2524.340
+0.8144 (sp 1.44
sp0.69
)
O
LP1(O23)
LP1(O25)
LP1(O27)
LP2(O23)
LP2(O27)
1.96023 -1633.294
1.96843 -1405.482
1.96963 -1825.304
-
-
-
-
-
-
-
-
-
-
sp2.02
sp0.75
1.87653
1.86146
-529.672
-736.32
sp99.99
0.13
0.17
99.70
99.62
sp99.99
0. 5492 (sp3.29
-0. 8357 (s)H
)
23.30 76.64
100
24.16 75.79
100
26.85 73.11
100
23.30 76.64
100
24.16 75.79
100
26.85 73.11
100
N
N
N
σ(N17-H18) 1.99274 -2121.574
σ(N17-H19) 1.99147 -2107.56
σ(N17-H20) 1.99092 -2106.754
69.84
70.80
72.79
30.16
29.20
27.21
30.16
29.20
27.21
69.84
70.80
72.79
-
0.8415(sp 3.14
)
+0.5403(s)H
-
0.8531(sp 2.72
+0.5217 (s)H
)
-
0. 5492 (sp3.29
-0. 8357 (s)H
0. 5403 (sp 3.14
-0. 8415 (s)H
0. 5217 (sp 2.72
-0. 8531 (s)H
)
N
σ∗ (N17-H18)
σ∗ (N17-H19)
σ∗ (N17-H20)
0.01085
0.00801
0.03039
1545.96
1590.446
1702.948
-
)
N
-
)
N
-

ACCEPTED MANUSCRIPT
Table 3. Second-order perturbation theory analysis of Fock matrix in NBO basis
ED(j)
(e)
E(j) − E(i)b F(I, j)c(arb.
Donor (i) ED(i) (e) Acceptor (j)
E(2)a(kJmol-1)
(arb. units)
units)
π(C2–C3)
π∗(C1–C6)
π∗(C4–C5)
0.31273
0.33411
78.162
84.63
0.30
0.30
0.067
0.070
1.68821
π∗(C1–C6)
π∗(C2–C3)
0.31273
0.33444
84.378
89.586
0.29
0.28
0.069
0.069
π(C4-C5) 1.65703
π(C1–C6) 1.66722
π∗(C2–C3)
π∗(C4–C5)
0.33444
0.33411
89.796
85.47
0.29
0.29
0.070
0.069
σ∗(C15-C21)
σ∗(C21-O23)
0.11178
0.05995
90.888
92.148
0.57
0.79
0.099
0.120
n2(O22)
1.86585
From unit 1 to unit 2
σ∗(H24-O25)
n1(O23)
1.96023
0.03149
38.514
1.45
0.103
From unit 2 to unit 1
n1 (O27)
n2(O27)
1.96963
1.86146
σ∗(N17-H20) 0.03039
15.12
1.36
0.94
0.062
0.062
σ∗(N17-H20)
0.03039
20.412
Within unit 2
n2(O25)
π∗(C26–O27)
π∗(C32–O33)
1.76508
1.81836
0.29420
0.24092
248.598
212.478
0.32
0.37
0.125
0.123
n2(O34)

ACCEPTED MANUSCRIPT
Table 4. NHO directionality and “bond bending” (deviations from line of nuclear centers)
Bond (A-B)
σ(C1-C2)
Deviation at A (°)
Deviation at B (°)
2.6
2.3
1.8
1.3
3.0
1.6
4.2
87.3
4.6
87.7
5.3
86.6
3.2
--
2.2
1.9
1.8
2.7
1.3
2.9
4
σ(C1-C2)
σ(C2-C3)
σ(C3-C4)
σ(C4-C5)
σ (C12–C15)
σ(C26-O27)
π(C26-O27)
σ(C32-O33)
π (C32-O33)
σ(C21-O22)
π (C21-O22)
σ (C21–O23)
σ (H24–O25)
σ (O25–C26)
87.5
3.8
87.7
4.6
87.6
2.9
5.1
4.0
3.9