J. Phys. Chem. A 2010, 114, 13055–13064
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Charge Transfer Interaction and Terahertz Studies of a Nonlinear Optical Material L-Glutamine Picrate: A DFT Study M. Amalanathan,† I. Hubert Joe,*,† and S. S. Prabhu‡ Centre for Molecular and Biophysics Research, Department of Physics, Mar IVanios College, ThiruVananthapuram-695 015, Kerala, India, and Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai-400 005, Maharashtra, India ReceiVed: August 6, 2010; ReVised Manuscript ReceiVed: October 29, 2010
Charge transfer interaction, vibrational spectra, and DFT computation of L-glutamine picrate has been analyzed. The equilibrium geometry, bonding features, and harmonic vibrational wavenumbers have been investigated with the help of B3LYP density functional theory method. The natural bond orbital analysis confirms the occurrence of strong intramolecular hydrogen bonding in the molecule. Terahertz time-domain spectroscopy was used to detect the absorption spectra in the frequency range from 0.025 to 2.8 THz. The vibrational modes found in molecular crystalline materials should be described as phonon modes with strong coupling to the intramolecular vibrations. 1. Introduction Terahertz waves are expected to be utilized for applications in ultra-high-capacity telecommunication, medical diagnosis, device inspection, seeing-through analysis, environmental sensing, etc. Terahertz (THz) radiation is located in the spectral region 0.1-10 THz (3-300 cm-1) between the microwave and mid-infrared region of the electromagnetic spectrum. The THz spectrum in the wavenumber range from 0.1 to 5 THz provides rich information on low wavenumber vibrational modes: crystalline lattice or intermolecular vibrational modes, hydrogenbonding stretches, and some torsion vibrations in many chemical and biological compounds, including small biomolecules, pharmaceutical materials, and explosives.1-4 The strength and nature of the electron donor-acceptor type bonding in the picric acid complexes are dictated by the nature of the partners involved in the bond-formation process.5 Consequently, most studies have focused on dipolar molecular systems in which the optical nonlinearities arise from intramolecular charge transfer.6,7 A second-order nonlinear optical (NLO) process can effectively generate THz waves. The NLO properties of materials in the visible and near-infrared region have been extensively studied since the development of the laser. Except for a few pioneering efforts,8,9 the far-IR region of the spectrum has remained largely unexplored. Coherent tunable terahertz waves have great potential for wavenumber domain spectroscopy and THz imaging applications.10,11 For efficient THz wave generation, the NLO crystal is required to have large nonlinear and low absorption coefficients. So, organic crystals with a large nonlinearity are promising candidates for wideband THz generation.12 In the past decades, density functional theoretical (DFT) calculation is an effective tool to predict the molecular structure, charge transfer interaction, IR, Raman bands, and inter- and intramolecular hydrogen bonding.13,14 In related context, that the computed results and experimental data were found to be in excellent agreement turned out to be fortuitous.15-20 In this * Corresponding author. Phone: +91 471 2531053. Fax: +91 471 2530023. E-mail:
[email protected]. † Mar Ivanios College. ‡ Tata Institute of Fundamental Research.
present study, the FT-IR, Raman spectra, and THz TDS studies of synthesized NLO material L-glutamine picrate (LGP) molecule using both theoretical and experimental techniques were analyzed and reported. This analysis was complemented by natural bond orbital (NBO) calculations with an analysis of the electron charge-transfer through the intramolecular contacts. The first-order hyperpolarizability calculation helps to confirm its NLO response. 2. Computational Density functional theory (DFT) calculations have been extensively used to predict the geometry and harmonic vibrational wavenumbers of organic compounds. The theoretical calculations were carried out using the Gaussian 03 program package21 for L-glutamine picrate. DFT calculations were used to determine the equilibrium structures and harmonic vibrational wavenumber of L-glutamine picrate. The Becke-3-Lee-Yang-Parr (B3LYP) method of DFT was used with the 6-31G (d) basis set.22,23 The computed B3LYP/ 6-31G(d) values fit with the experimental values better than the other methods.24 The optimized geometry corresponding to the minimum on the potential energy surface has been obtained by solving the self-consistent field equation iteratively. The harmonic vibrational wavenumbers have been analytically calculated by taking the second-order derivative of energy using the similar level theory. The calculated vibrational wavenumber was scaled down by using the scaling factor 0.961425 to offset the systematic error caused by neglecting anharmonicity and electron density. IR intensities and Raman activities have also been calculated. The calculated Raman activities (Si) have been converted to relative Raman intensities (Ii) using the following relationship derived from the basic theory of Raman scattering.26,27
Ii )
f(νo - νi)4Si -hcνi νi 1 - exp kT
[
(
)]
(1)
where νo is the exciting wavenumber, νi is the vibrational wavenumber of the ith normal mode, h, c, and k are universal
10.1021/jp107414x 2010 American Chemical Society Published on Web 11/24/2010
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Amalanathan et al. size 2 mm is placed in the THz optical path, and the waveforms are recorded. The numerical Fourier transforms of the pulses were taken and the additional absorption dips due to the presence of the samples were found. All measurements have been done at room temperature. The range of the THz for this experiment is 0.025-2.8 THz (limited by detector ZnTe). Figure 1 is a schematic illustration of the terahertz experimental set up. 4. Optimized Geometry
Figure 1. Experimental setup for the THz spectrometer.
constants, and f is the suitably chosen common scaling factor for all the peak intensities. The vibrational modes were assigned on the basis of TED analysis using the program VEDA.28 3. Experimental Methods 3.1. Growth and IR and Raman Spectra. The compound L-glutamine and picric acid were taken in 1:1 stoichiometric ratio in deionized double distilled water to get a saturated aqueous solution and was then allowed to evaporate slowly at room temperature. A crystal was obtained within 2 weeks. Repeated recrystallization yielded good quality L-glutamine picrate (LGP) crystals. The NIR FT-Raman spectrum of LGP was recorded using a Nicolet Nexus spectrometer in the region 3700-100 cm-1. The FT-IR spectrum of LGP was obtained in the region 4000-500 cm-1 recorded using a Nicolt Magna 560 FT-IR spectrometer. 3.2. THz Measurement. The THz measurements were performed using a standard THz time domain spectrometer (TDS). The THz beam is generated from an antenna structure put over low-temperature-grown GaAs by gating it with 10 fs pulses (repetition rate 76 MHz) having 780 nm center wavelength. The generated THz pulses were transmitted through a standard aperture and the pulse waveform. A grown crystal of
Figure 2. Optimized structure of LGP calculated at B3LYP/6-31 G(d).
The optimized structure of LGP determined using DFT method is shown in Figure 2. The optimized geometrical parameters are summarized in Table S1 (Supporting Information). From the geometry, the calculated bond length of C6-O7, C6-O8, and C2-O11 are 1.2043, 1.357, and 1.248 Å, respectively. These bond length differences in the C-O groups are due to different environments of oxygen. The decrease in endocyclic angle C22-C23-C24 (111.71°) and increase in excocylic angles O28-C23-C22 (123.31°) and O28-C23-C24 (124.79°) shows the possibility of hydrogen bonding in that ring moiety. The variation in dihedral angles C25-C24-N29-O30 (-8.72°), C25-C24-N29-O31 (171.38°), C21-C22-N35-O36 (-161.08°), and C21-C22-N35-O37 (18.06°) than the other (0.65°) and dihedral angles C25-C20-N32-O33 C25-C20-N32-O34 (-179.29°) is due to charge delocalization from NH3 to NO2 group and the steric repulsion between O26 and O36, respectively. The weakening of N15-H16, N15-H17, and N1-H18 bonds than the other N-H bond is caused by the intramolecular hydrogen bonding in the molecule. The N-H · · · O hydrogen bond geometry of LGP is given in Table S2 (Supporting Information). The lengthening of bond distances C23-C24 (1.479 Å) and C23-C22 (1.569 Å) and contraction of the internal bond angle C24-C23-C22 (111.7°) clearly shows hyperconjugation (n fσ*) between lone-pair electrons of O28 atom and C22-C23 and C23-C24 bonds. 5. NBO Analysis The natural bond orbital (NBO)29 analysis has proved to be an effective tool for chemical interpretation of hyperconjugative interaction and electron density transfer from the filled lone pair electron. The DFT/B3LYP level has been used in order to investigate the various second-order interactions between the filled orbitals of one subsystem and vacant orbitals of another subsystem, which is a measure of the delocalization or hyperconjugation.29 The hyperconjugative
DFT Study of L-Glutamine Picrate
J. Phys. Chem. A, Vol. 114, No. 50, 2010 13057
TABLE 1: Second-Order Perturbation Theory Analysis of Fock Matrix in NBO Basis Corresponding to the Intramolecular N-H · · · O Hydrogen Bonds of LGP donor NBO
acceptor NBO
E(2) (kJ/mol)
E(j) - E(i)
F(i,j)
LP1 O11 LP2 O11 LP1 O28 LP2 O28 LP1 O30 LP2 O30
σ*(N15-H17) σ*(N15-H17) σ*(N1-H18) σ*(N1-H18) σ*(N15-H16) σ*(N15-H16)
1.53 0.68 2.08 0.15 1.73 4.19
1.04 0.60 1.17 0.73 1.13 0.66
0.036 0.018 0.044 0.010 0.040 0.048
interaction energy was deduced from the second-order perturbation approach.
(2)
E
Fij2 〈σ|F|σ〉2 ) -nσ ) -nσ εσ* - εσ ∆E
(2)
where 〈σ|F|σ〉 or Fij is the Fock matrix element between i and j NBO orbitals, εσ and εσ* are the energies of σ and σ* NBO’s, and nσ is the population of the donor σ orbital. In this work, NBO’s were applied for the evaluation of the hydrogen bond in LGP. Tables 1 and 2 show some of the significant donor-acceptor interactions and their second-order perturbation energies E(2). In the NBO analysis of the hydrogenbond system, the charge transfer between the lone pairs of proton acceptor and antibonds of the proton donor is the most important. The LGP molecule exhibits red-shifted N-H · · · O hydrogen bonds. The importance of hyperconjugation interaction and electron density transfer (EDT) from lone electron pairs of the Y atom to the X-H antibonding orbital in the X-H · · · Y system are well-documented.30 In general, such interactions lead 2
2
to an increase in population of X-H antibonding orbital, which elongates the X-H bond. On the basis of the theoretical analysis, the optimized geometry shows the elongation of the N-H bond. The NBO analysis clearly explains the evidence of the formation of strong H-bonded interaction between LP(O) and σ*(N-H) antibonding orbital. The stabilization energy E(2) associated with hyperconjugative interaction σ*(N15-H17), σ*(N1-H18), and σ*(N15-H16) are 6.4, 8.7, and 7.2 kJ mol-1, respectively. This is due to the accumulation of the electron density in the N-H bond drawn not only from n(O) of the hydrogen acceptor but also from the entire molecule, leading to its elongation and concomitant red shift of the N-H stretching wavenumber.31
6. First Hyperpolarizabilities The first-hyperpolarizability (β0) and its related properties (β, R0, and ∆R) have been calculated on the basis of the finite field approach. In the presence of an applied electric field, the energy of a system is a function of the electric field and the first hyperpolarizability is a third rank tensor that can be described by a 3 × 3 × 3 matrix. The 27 components of the 3D matrix can be reduced to 10 components because of the Kleinman symmetry.32 The matrix can be given in the lower tetrahedral format. It is obvious that the lower part of the 3 × 3 × 3 matrixes is a tetrahedral. The components of β are defined as the coefficients in the Taylor series expansion of the energy in the external electric field.32 When the external electric field is weak and homogeneous, this expansion becomes
TABLE 2: NBO Result Showing the Formation of Lewis and Non-Lewis Orbitals by the Valence Hybrids Corresponding to the Intramolecular N-H · · · O bond (A-B)
ED (au)
energy
σ(N15-H16)
1.99130
-0.7477
σ*(N15-H16)
0.02586
0.33459
σ(N15-H17)
1.98927
-0.74566
σ*(N15-H17)
0.04696
0.32337
σ(N1-H18)
1.98267
-0.67415
σ*(N1-H18)
0.01494
0.49649
σ(C2-O11)
1.99507
-1.02337
σ*(C2-O11)
0.02089
0.49502
σ(C23-O28)
1.96978
-1.01861
σ*(C23-O28)
0.00967
0.01280
σ(N29-O30)
1.99488
-1.11693
σ*(N29-O30)
0.05372
0.42308
LP1 O11
1.97256
-0.71503
LP1 O28
1.97519
-0.66926
LP1 O30
1.97669
-0.79839
EDA (%) 75.03 24.97 76.67 23.33 72.37 27.63 34.68 65.32 31.50 64.30 47.88 52.12 -
EDB (%) 24.97 75.03 23.33 76.67 27.63 72.37 65.32 34.68 68.50 35.70 52.12 47.88 -
-
-
-
-
-
-
NBO 3.23
0.8662(sp )N +0.4997 (s)H 0.4997(sp3.23)N -0.8662 (s)H 0.8756(sp2.88)N +0.4830 (s)H 0.4830(sp2.88)N -0.8756 (s)H 0.8507(sp2.40)N +0.5256 (s)H 0.5256(sp2.40)N -0.8507 (s)H 0.5889(sp2.29)C +0.8082(sp1.59)O 0.8082(sp2.29)C -0.5889(sp1.59)O 0.5613(sp99.99)C +0.8276(sp99.99)O 0.8019(sp2.22)C -0.5975(sp1.46)O 0.6919(sp2.29)N +0.7220(sp2.74)O 0.7220(sp2.29)N -0.6919(sp2.74)O sp0.64 sp0.68 sp0.39
s (%)
p (%)
23.65 23.65 25.77 25.77 29.44 29.44 30.35 38.50 30.35 38.50 0.01 0.02 30.99 40.55 30.38 26.69 30.38 26.69
76.30 76.30 74.19 74.19 70.52 70.52 69.54 61.19 69.54 61.19 99.84 99.72 68.92 59.16 69.51 73.18 69.51 73.18
61.04 59.40 72.16 -
38.92 40.57 27.83 -
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E ) E°-µiFi - 1/2RijFiFj - 1/6βijkFiFjFk i j k l
1/24γijklF F F F
TABLE 3: First-Order Hyperpolarizibility (β) of LGP Derived from DFT Calculations
(3)
where Eo is the energy of the unperturbed molecules, Fi is the field at the origin µi, Rij, βijk are the components of dipole moment, polarizabiltiy and the first hyperpolarizability, respectively. From this the first hyperpolarizabilities (β0) using the x, y, z component is defined as
β0 ) (βx2 + βy2 + βz2)1/2
(4)
The distortions are the result of substitution effect of the NO2 and C-O groups joined to the ring. The electron-withdrawing NO2 group at the ring can produce a decrease of the electron density (ED) due to intramolecular charge transfer (ICT) in the conjugated ring system, which reduces the double bond character. The calculated first-order hyperpolarizibility (β) is given in Table 3. The theoretical calculation seems to be more helpful in the determination of particular components of β tensor than in establishing the real values of β. Domination of particular components indicates on a substantial delocalization of charges in those directions. It is noticed that in the βxxy (7.16 × 10-29 esu) direction, the biggest values of hyperpolarizability are noticed, and subsequently, delocalization of the electron cloud is more in that direction. This is the indication of the nonlinear polarization induced by the component and this particular tensor component is responsible for the THz application in LGP crystal. The calculated first hyperpolarizability of LGP is 7.36 × 10-30 esu, which is 27 times that of urea,33,34 which is substantial for the NLO property and the possibility of THz emission in the molecule.
β components
values (esu)
βxxx βxxy βxyy βyyy βzxx βxyz βzyy βxzz βyzz βzzz βtotal
5.612 × 10-29 7.16 × 10-20 -8.64 × 10-30 -6.34 × 10-30 -3.23 × 10-30 1.05 × 10-29 7.84 × 10-30 -9.63 × 10-30 -7.82 × 10-30 5.46 × 10-30 7.36 × 10-30
shape are due to absorption by low-frequency phonons modes. The terahertz radiation transmitted (Figures 3 and 4) from the grown crystal shows that we can use this crystal for the terahertz imaging or the other terahertz application in the narrow THz range of higher transmission. 8. Vibrational Spectral Analysis The vibrational spectral analysis is performed on the basis of characteristic vibrations of phenyl ring, carbonyl, amine, and methelene groups separately. The ring group vibration comprises the C-H and C-C modes. The observed and simulated FT-IR and Raman spectra are shown in Figures 5 and 6, respectively.
7. Terahertz Study The THz-field transmitted through a sample is modified by dispersion and the absorption of the media under examination. The ratio of the electric field strength before (Er) and after transmission (Es) is given by
{
Es inωd ) T(n) exp -Rd + Er c
}
(5) Figure 3. Transmission of the THz pulse in the spectral domain.
where d is the thickness of the sample, ω the frequency of the radiation, c the speed of light in vacuum, and T(n) is the reflection loss at the sample surface, R is the power of absorption coefficient, and n is the refractive index.35 The spectral-domain spectra of the reference passing through nitrogen directly and the samples passing through LGP are shown in Figure 3. THz pulse passing through nitrogen is shown in Figure 3a (reference) and Figure 3b (samples). It is clear that the THz pulses are delayed due to the refractive index of the samples and attenuated due to their absorption and the Fresnel transmission coefficients.36 The collective data of THz are not considered due to interference between reflections of the probe pulse inside the sample pellets, and a small increase in background with frequency is shown in the absorption spectra and would result from scattering of the samples. The THz spectral domain spectra of LGP are shown in Figure 4. The low value absorption peak in LGP is due to intermolecular phonon modes present in the LGP. One can have the production of THz pulses by exciting the molecular organic crystals due to electron transfer within the constituent molecules. The remarkable variations in band
Figure 4. FFT amplitude of the terahertz from LGP.
DFT Study of L-Glutamine Picrate
Figure 5. (a)FT-IR spectra of LGP and (b) simulated IR spectra of LGP.
Figure 6. (a) FT-Raman spectra of LGP and (b) simulated Raman spectra of LGP.
The calculated vibrational wavenumbers and measured infrared band position and their tentative assignments are given in Table 4. 8.1. C-H Vibrations. The heteroaromatic structure shows the presence of C-H stretching vibration in the region 3080-3010 cm-1, which is the characteristic region for the identification of the C-H stretching vibration.37 In this region the bands are not affected appreciably by the nature of substituents. The observed medium band in IR at 3085 cm-1 and the weak band in Raman at 3080 cm-1 are assigned to the C-H stretching mode of the ring. The in-plane and out-of-plane aromatic C-H bending vibrations are expected in the region 1290-1000 and 900-675 cm-1. In LGP, the C-H in-plane bending mode vibration is observed in Raman at 1290 cm-1 and in IR at 1246 cm-1 as medium shoulder intensity; the increase in intensity is due to the polar ring substituents present in LGP. The C-H out-of-plane bending mode vibrations are observed as a weak band in IR at 936 cm-1 and a medium band in Raman at 938 cm-1. The observed wavenumbers gets blueshifted from the expected value due to the highly electronegative nitro group present in the ring. The observed bands in IR at 909 and 831 cm-1 and Raman at 823 cm-1 are also assigned C-H out-of-plane bending vibration. 8.2. C-C Vibrations. The aromatic ring carbon-carbon stretching vibrations occur in the region 1626-1430 cm-1.34 The fifth ring stretching vibration is active near 1315 ( 65 cm-1, a region that overlaps strongly with that of the CH in-plane
J. Phys. Chem. A, Vol. 114, No. 50, 2010 13059 deformation.37 The series bands observed in the IR at 1567 (s) and 1547 (s) cm-1 and in Raman at 1570 (w) and 1560 (s) cm-1 are assigned to the C-C stretching vibration. The simultaneous activation of ring C-C stretching mode provides apparent evidence for the charge transfer interaction.38,39 Most of the C-C stretching vibrations are observed between 1625 and 1540 cm-1 with increasing intensity, which shows that the substituents CdO and NO2 are directly conjugated with the ring. The aromatic ring deformation vibrations are expected in the region around 505-585 cm-1.40 The ring deformation of LGP is observed in IR at 525 cm-1. 8.3. CdO and Carboxylic Group Vibration. The characteristic infrared absorption frequencies of carbonyl group in cyclic ketones have been investigated.39,41 The CdO stretching vibration band can be easily identified from the IR, and because of the degree of conjugation, the strength and polarizations are increased. The carbonyl stretching vibrations in ketones are expected in the region 1715-1680 cm-1.41 The carbonyl C23-O28 stretching vibration is observed as a medium band in IR at 1605 cm-1. The observed wavenumber lowering from the computed value in LGP is due to the presence of a higher degree of conjugation and the intramolecular hydrogen bonding within (N1-H18 · · · O28). In general, the CdO stretching vibration for the carboxylic acid gives rise to a band that is stronger than that of ketones or aldehydes. In the solid phase, the CdO group of saturated aliphatic carboxylic acid absorbs strongly in the region 1740-1715 cm-1. The carboxyl C6-O7 stretching mode is observed as a medium band in IR at 1623 cm-1 and in Raman at 1610 cm-1. As a result of hydrogen bonding, carboxylic acid in liquid and solid phases exhibits a broad intense band at 3300-2500 cm-1 due to the O-H stretching. The observed medium band in IR at 3291 cm-1 is assigned to the O-H stretching mode of the carboxylic acid. 8.4. Nitro Group Vibrations. The NO2 asymmetric and symmetric stretching vibration are expected in the range 1625-1540 and 1400-1360 cm-1,37 respectively. Usually, the symmetric vibration is stronger than the asymmetric one in the infrared.42,43 This could be due to the electron-withdrawing substituent adjacent to the nitro group tending to increase the wavenumber of asymmetric vibration and decrease that of the symmetric vibration. The strong IR band observed at 1547 cm-1 has been assigned to the NO2 asymmetric stretching vibration. The observed very strong, intense band at 1336 cm-1 in IR and 1340 and 1310 cm-1 in Raman are assigned to NO2 symmetric stretching vibrations, respectively. The intensity enhancement of these wavenumbers is due to conjugation with the aromatic ring.41 The NO2 deformation vibration includes NO2 scissoring (in-plane), NO2 wagging (out-of-plane), and NO2 rocking (inplane) usually occur at wavenumbers below 900 cm-1.41,44,45 The NO2 deformations occur in IR at 796 and 704 cm-1 and Raman at 703 cm-1. The NO2 deformation that occurs at higher wavenumber is due conjugation of CdC.41 The NO2 group oxygen forms bifurcated hydrogen bonding with an NH3 group of the neighboring molecule. The shortest intermolecular distance between the NO2 group oxygen and the NH3 group nitrogen N-H · · · O is 2.851 Å. The hydrogen bonds most often found in nonlinear optical crystals of organics involve an interaction between a hydrogen bond to sp3 nitrogen or oxygen and an oxygen atom with more s-bond character, although more symmetrical interactions also occur. The amino group in picric acid might be far from planar and that the amino nitrogen can act as a hydrogen-bond acceptor in the present system. The observed important nonplanarity of the amino group
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TABLE 4: Vibrational Assignment of LGP at the DFT Level νcal (cm-1)
IR int
Raman int
νIR (cm-1)a
3545 3489 3353 3282 3141 3135 3019 3008 2999 2959 2943 2908 2814 1799 1687
114.7 58.9 154.9 534.7 12.2 12.4 2.1 2.3 13.6 11.6 6.7 1509.6 616.9 248.6 73.4
4.95 1.7 1.63 3.61 0.92 1.87 2.61 1.74 4.26 5.96 5.06 23.09 3.27 1.85 0.41
3297 m 3260 m 3155 b 3085 m 3021 m 2830 w 1623 m -
3250 vw 3080 vw 3046 vw 2996 vw 1610 vw -
O-H stretching NH2 asym stretching NH3 asym stretching NH2 sym stretching C-H stretching of ring C-H stretching of ring CH2 asym stretching of C4, C3 CH2 asym stretching of C3, C4 C5-H14 stretching CH2 sym stretching of C4 CH2 sym stretching of C3 NH3 sym stretching NH3 sym stretching C6dO7 stretching C2dO11 stretching + NH2 scissoring
1668 1638 1624
154.1 413.3 457.8
1.45 1.95 1.08
1605 s -
-
1597 1584
82.4 202.1
0.56 8.51
1574 s -
1570 w
NH3 asym bending C23dO28 + NH3 asym bending NH3 asym bending + NH2 scissoring NH3 asym bending CdC stretching in rings
1566
85.8
12.4
1567 s
1560 s
CdC stretching in ring
1561 1543 1504
49 156.2 0.9
6.04 0.9 20.47
1547 s 1505 m 1489 m
1500 w -
1453 1451
89.6 2.2
1.92 2.47
1456 w
1450 w
NO2 asym stretch of N35 NH3 umbrella mode C-C stretching of the ring + NH3 umbrella mode CH2 scissoring CH2 scissoring of C3, C4
1433 1414
39.3 55.6
3.86 7.7
1425 vs -
1413 1361
43.7 0.6
11.83 1.94
1364 m
1350 1343
134.8 2.2
93.9 1.62
-
1340 vvs -
1333
33.7
25.32
-
-
1326 1309
606.3 396.5
65.84 20.03
1336 vvs -
1310 vs -
1307
190.6
7.29
-
-
1301 1276
247 3.4
30.65 0.27
-
1290 ms -
1252 1247
150.8 689.6
7.78 14.11
1246 m sh 1243 vs
-
1207
13.5
4.1
-
-
1166
89.2
1
-
-
1151
37.5
3.26
1141 1134
53.9 17.9
0.78 0.6
1107
56.2
0.85
1095 1057
275.2 80.7
1.38 4.85
1047
28.2
0.64
1162 m -
νRaman (cm-1)a
1363 m
1160 m
assignmentb
CH2 scissoring of C3 C-N stretching + CH2 waggging of C3 CdC stretch in ring CH2 wagging of C3 + CH2 twisting of C4 NO2 sym stretch C-H bending + O-H bending C-H ipb in the ring NO2 sym stretch CH2 wagging of C3 + CH2 twisting of C4 O-H ipb + C-H ipb C-H ipb of the ring CH2 wagging of C3 + CH2 twisting of C4 CH2 twisting of C4 CH2 wagging of C3 + CH2 twisting of C4 + NH2 wagging CH2 twisting of C4 +CH2 wagging of C3 CH2 twisting of C3 + CH2 wagging of C4 12 of ring
-
CH2 wagging of C4 C-H ipb of the ring
1084 w
-
NH2 rocking
1053 vw
1090 w 1050 vw
-
-
O-H ipb + NH2 rocking C-H ipb 18a C5-N15 stretch
PED [%]c νO8-H38 [100] νasNH2 [92] νasNH3 [99] νsNH2 [92] νC25-H27 [99] νC21-H26 [100] νasMe2 [79] + νasMe1 [13] νasMe1 [76] + νasMe2 [12] νC5-H14 [90] νsMe2 [95] νsMe1 [89] νsN6 [96] νsNH3 [95] νC6-O7 [88] νC2-O11 [31] + νC2-N1 [14] + δH18-N1-H19 [15] + δH16-N15-H17 [21] δasNH3 [58] νC23-O28 [39] + δasNH3 [15] δsNH3 [36] + δscisNH2 [22] δasNH3 [17] νC20-C25 [15] + νC23-O28 [10] + νN32-O34 [19] +νN32-O33 [22] νC21-C22 [18] + νC23-O28 [17] + νN32-O34 [19] + νN32-O33 [22] νasN(35)O2 [73] δH17-N15-H39 [16] νN32-O34 [10] + νC20-C25 [15] + νN32-O33 [22] δH9-C4-H10 [25] δH12-C3-H13 [12] + δH9-C4-H10 [39] δH12-C3-H13 [47] νC2-N1 [15] + δH12-C3-H13 [15] νC2-C3 [11] + νC21-C22 [12] δH9-C4-H10 [16] + δC3-C4-H10 [10] νasN(32)O2 [37] + νasN(35)O2 [25] δC3-C4-H10 [11] + τO8-C6-C5-H14 [35] δC20-C21-H26 [20] + νN32-O34 [15] νasN(32)O2 [22] + νasN(35)O2 [34] δC3-C4-H10 [14] δOH [22] + δO7-C6-O8 [11] + νC6-O8 [14] δCH [12] δC4-C3-H13 [12] + δC3-C4-H10 [37] δC3-C4-H10 [37] δC4-C3-H13 [12] + δC3-C4-H10 [37] δC4-C3-H13 [56] + τN1-C2-C3-H13 [12] δC6-O8-H38 [13] + δC3-C4-H10 [18] δC22-C23-O28 [16] +δC20-C21-C25[12] + νC24-N29 [10] δC3-C4-H10 [21] νC22-C23 [17] + δC20-C21-H26 [19] + δC20-C25-H27 [22] νC2-N1 [10] + τO28-H18-N1-H19 [11] δOH [21] + δC3-C4-H10 [11] δC20-C21-H26 [25] + δC20-C25-H27 [24] νC4-C5 [28] + νC5-N15 [21] + νC3-C4 [20]
DFT Study of L-Glutamine Picrate
J. Phys. Chem. A, Vol. 114, No. 50, 2010 13061
TABLE 4: Continued νcal (cm-1)
IR int
Raman int
νIR (cm-1)a
νRaman (cm-1)a
1014 987
5 19
0.96 2.75
-
-
937 929
8.8 12.9
1.29 4.26
936 w -
938 m -
926 899 897 864
20.5 26.4 11.2 4.2
1.82 7.66 8.69 1.76
909 m -
-
811
0.1
0.75
831 vw
823 vw
806 794
21.2 6.5
1.12 39.51
796 m
-
C-O-H bend NO2 bending
769
20
2.54
-
-
assignmentb C3-C4 stretch C5-N15 stretch + CH2 twist of C3+ CH2 rocking of C4 C-H opb of ring C20-N32 stretch C-H opb of ring CH2 rocking of C3, C4 C22-N35 stretch CH2 rocking of C4 C-H opb of ring
767
7.7
5.61
744 m
751 w
749 723
0.9 21.5
0.95 1.07
733 w
-
C-H opb of ring +CH2 rocking of C4 + NH2 rocking CH2 rocking of C4 + NH2 rocking C22-N35 bending C20-N32 bending
705
32.4
5.23
712
-
C24-N29 C22-N35 bending
697
59.6
1.99
704 m
692 675
79.7 22.9
3.49 3.5
-
673 657
6 172.9
1.74 2.25
657 w
643
181.9
3.22
-
-
OH opb
620
140.7
1.86
-
-
591 547
13.1 51
1.69 5.82
545 w
545 w
C-O bending + C-O-H bending C2-O11 bending C6-O8 bending + NH2 wagging
530
7.6
1.51
525 w
-
ring deformation
511
2.5
1.2
-
-
508 505
8.1 16
5.35 3.55
-
506 w -
CH2 rocking of C3 + NH2 rocking NH2 rocking NH3 torsion
484
66.9
1.48
-
-
NO2 rocking
478
1.4
1.37
-
-
NO2 rocking + C-H opb of ring
457
2.7
2.06
-
443 w
ring torsion
438
2.6
0.25
419 w
NH3 torsion
393
45.4
0.63
-
-
NO2 torsion of N29
380
1.2
5.08
-
-
CdO torsion + NH3 torsion
370
19.3
20.25
-
-
NO2 torsion of N35
350
22.9
3.71
-
-
C-N torsion
337 323
1.8 2.7
5.27 19.36
-
337 w -
308
1.9
4.48
-
-
m
428 vw
703 w 658 vw
NO2 deformation of N29, N35 C-O bending NO2 deformation of N32 + NH2 twisting NH2 wagging NH2 wagging
CH2 torsion + NH2 torsion ring torsion C-N torsion
PED [%]c νC5-C6 [31] νC5-N15 [15] + δC3-C4-H10 [12] + τN1-C2-C3-H13 [10] τN32-C20-C21-H26 [75] νC20-N32 [16] + τN32-C20-C25-H27 [22] τN32-C20-C25-H27 [61] τC2-N1-C3-O11 [10] νC22-N35 [14] νC2-C3 [13] + νC5-C6 [20] + νC4-C5 [15] δC20-C21-C22 [11] + δO34-N32-O33 [17] + δC20-C21-C25 [21] + δC21-C22-C23 [13] τC6-O8-C5-O7 [17] δO34-N32-O33 [17] + δO37-N35-O36 [21] + δO31-N29-O30 [16] τO37-C22-O36-N35 [14] νC2-C3 [31] τO37-C22-O36-N35 [30] τN32-O33-C20-O34 [69] + τC20-C21-C25-N32 [11] δO34-N32-O33 [13] + δO31-N29-O30 [13] δC20-C21-C22 [11] + δO37-N35-O36 [21] + δO31-N29-O30 [17] τC6-O8-C5-O7 [25] δO34-N32-O33 [10] + τC3-C2-N1-H18 [22] τN1-C2-C3-H18 [22] τO28-H18-N1-H19 [12] + τC3-C2-N1-H18 [12] τO28-H18-N1-H19 [14] + τC3-C2-N1-H18 [38] δO7-C6-O8 [17] + τC5-C6-O8-H38 [45] δN1-C2-O11 [39] δO7-C6-O8 [21] + τC5-C6-O8-H38 [25] τC20-C21-C22-C25 [11] + δC20-N32-O33 [54] τC20-C21-C22-C25 [11] + τC20-C21-C24-C25 [30] τC3-C2-N1-H18 [12] δC5-C6-O8 [27] + τC4-C5-C6-N15 [10] νC5-C6 [11] + δO7-C6-O8 [13] + δC3-C2-N1 [14] + δC2-C3-C4 [11] δC22-N35-O36 [36] + δC24-N29-O30 [17] δC22-C23-O28 [13] + δC24-N29-O30 [10] δC4-C5-N15 [11] + τO30-H16-N15-H17 [13] δC3-C2-N1 [15] + δC4-C5-N15 [30] δC22-C23-O28 [21] + δC22-N35-O36 [12] + τC20-C21-C24-C25 [10] νC24-N29 [10] + δC22-C23-O28 [13] + δC20-N32-O33 [10] δC5-C6-O8 [29] ) τC4-C5-C6-N15 [18] νC20-N32 [36] νC22-N35 [26] + δC20-C21-C25 [13] + δC21-C22-C23 [13] δC3-C2-N1 [17] + δC3-C4-C5 [29]
13062
J. Phys. Chem. A, Vol. 114, No. 50, 2010
Amalanathan et al.
TABLE 4: Continued νcal (cm-1)
IR int
νIR (cm-1)a
νRaman (cm-1)a
304
4.5
1.56
-
-
CH2 wagging of C3, C4
289 250
2.3 8.1
4.51 1.62
-
-
NH3, NH2 torsion
224 197
43.2 4.9
4.75 1.79
-
207 w
189
10.3
5.56
-
-
COOH torsion + NO2 torsion
186
1
4.7
-
-
ring torsion
163
7.3
4.53
-
175 w
COOH torsion + N15-H16 · · · O30 stretch
146
7.7
3.94
-
-
CdO torsion + NH · · · O stretch
142 120 107
10 4.7 17.3
7.16 1.49 8.73
-
-
NO2 torsion of N35 + NH2 torsion NH2 torsion + ring torsion NO2 torsion of N29 + CH2 torsion
87
2.3
7.3
-
-
75
3.9
5.99
-
-
NO2 torsion N29, N32, N35 + N15-H16 · · · O30 stretch C-N torsion
63
1.9
7.34
-
-
NO2 torsion of N29
60 54 48 41 30 24
0.5 0.8 2.4 1.4 0.5 0.1
19.2 8.87 19.87 11.85 12.53 55.46
-
-
15
0
-
-
NO2 torsion of N32 NO2 torsion of N35 NO2 torsion of N35 COOH torsion CH2 torsion NO2 torsion + CdO torsion + NH3 torsion NO2 torsion + CdO torsion + NH3 torsion
Raman int
100
assignmentb
NO2 torsion of N29, N32, N35 NO2 torsion of N29, N32, N35
PED [%]c δC22-N35-O36 [10] + δC24-C25-N29 [18] + δC20-C25-N32 [11] + δC21-C22-N35 [11] τC20-C21-C22-C23 [10] δC3-C2-N1 [14] + δC2-C3-C4 [37] τC2-C3-C4-C5 [32] δC4-C5-C6 [11] + τC23-C24-N29-O30 [12] δC4-C5-C6 [21] + τC20-C21-C22-C25 [17] δC24-C25-N29 [17] + δC21-C22-N35 [12] νN15-H16 · · · O30 [14] + δC20-C25-N32 [25] + δC4-C5-C6 [11] τN29-O30-H16-N15 [19] + τC5-N14-H16-O30 [15] δC21-C22-N35 [26] τC24-N29-O30-H16 [13] δH16-O30-N29 [15] + δC3-C4-C5 [12] νH16 · · · O30 [16] + τN1-C2-C3-C4 [16] δN15-H16-O30 [15] + τC3-C4-C5-N15 [20] τC21-C20-N35-O36 [25] + τN1-C2-C3-C4 [17] τC21-C20-N35-O36 [59] τC21-C22-N35-O36 [50] τC21-C22-N35-O36 [34] τN15-C5-C60-O8 [69] δH16-O30-N29 [24] τC24-N29-O30-H16 [21] δN15-H16-O30 [18] + τC24-N29-O30-H16 [24]
a
vvs, very, very strong; vs, very strong; s, strong; m, medium; w, weak; vw, very weak; b ipb, in-plane bending; opb, out-of-plane bending. ν, stretching; νs, symmetric stretching; νas, asymmetric stretching; δ, bending; δs, symmetric bending; δas, asymmetric bending, δscis, scissoring; τ, torsion. c
and its capability to work as a hydrogen-bond acceptor in LGP indicate that the amino nitrogen atom presents a high degree of sp3 character. 8.5. Methylene Group Vibration. The methylene group vibrations are assigned on the basis of the spectral similarity to the related amino acid compounds. In amino acids, the CH2 asymmetric and symmetric vibrations are expected to occur in the regions 3100-3000 and 3000-2900 cm-1, respectively.46 The observed medium band in IR at 3021 cm-1 and the weak band in Raman at 3046 cm-1 are assigned to asymmetric stretching of CH2. Electronic effects including back-donation, mainly caused by the presence of nitrogen atom adjacent to methylene groups, can shift the position and alter the intensity of CH stretching and bending modes. The scissoring mode of the CH2 group gives rise to a characteristic band near 1420-1412 cm-1. The scissoring mode of the CH2 group is observed in IR at 1456 cm-1 and in Raman at 1450 cm-1 as a weak band. The rocking and wagging vibrations appear in the region of 1309-1288 and 910-905 cm-1, respectively. The observed CH2 deformations are given in Table 4. 8.6. Amino group Vibration. In saturated amines, the asymmetric NH2 stretching and their symmetric counter parts are usually expected in the region 3380-3350 and 3310-3280 cm-1, respectively.47,48 However, the protonation of the NH2 group can shift the band position toward the range 3300-3100 and 3100-2600 cm-1 for asymmetric and symmetric stretching
modes, respectively. The NH3+ stretching bands are broader and weaker in IR than those arising from the uncharged NH. groups. The asymmetric stretching modes of NH2 are observed in IR at 3260 cm-1 and Raman at 3250 cm-1, and NH3+ is observing in IR at 3155 cm-1. Furthermore, the lowering of NH3+ symmetric stretching mode band position and broadness indicates the formation of intramolecular N-H · · · O hydrogen bonding. This type of hydrogen bonding is believed to be the source of nonlinearity in crystals like urea and methyl 2,4-dinitrophenylaminopropanate.49 It is already established that the molecular hyperpolarizability and mechanical stabilities get enhanced in organic molecules containing OH and NH groups, which are involved in hydrogen-bond interactions.50 This intramolecular hydrogen bonding is responsible for the high β value, and this mechanism plays an important role in the NLO property. The NH3+ asymmetric and symmetric bending vibrations are generally expected near 1660-1610 and 1550-1485 cm-1. The observed strong band in IR at 1574 cm-1 and medium band at 1504 cm-1 are assigned for asymmetric and symmetric deformations of NH3+. 8.7. Skeletal Modes. The absorption bands arising from C-N and C-C stretching vibrations are generally measured in the region 1150-850 cm-1.51 The C4-C5 stretching mode is observed as a weak band in IR at 831 cm-1. Likewise, the C-N stretching modes are observed at 733 and 712 cm-1.
DFT Study of L-Glutamine Picrate
J. Phys. Chem. A, Vol. 114, No. 50, 2010 13063 10. Conclusion Vibrational spectral analysis of LGP was performed using density functional theory calculations. The NBO analysis obviously exhibits the formation of N-H · · · O intramolecular hydrogen bonding. Second-order perturbation theory results show that the transfer of ED from the LPO to the σ*(N-H) bond make strong evidence of N-H · · · O hydrogen bonding. The lowering of NH3+ symmetric stretching wavenumber confirms the N-H · · · O hydrogen bonding. The simultaneous occurrence of ring C-C mode vibration provides evidence for the charge transfer interaction, which is responsible for the NLO property. The calculated first hyperpolarizability of LGP is found to be 7.36 × 10-30 esu, which is 27 times that of urea. The high β value and low HOMO-LUMO energy gap value are an additive property to exhibit nonlinear optical property. Hence, the present NLO crystal can be a better entrant for the THz application. Acknowledgment. I.H.J. thanks the University Grants Commission (UGC), New Delhi, India, for the financial support (Grant no: F.No.35-2/2008). Supporting Information Available: Hydrogen-bond geometry of LGP and the calculated hyperpolarizability values. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes
Figure 7. (a) HOMO and (b) LUMO plot of LGP.
9. HOMO-LUMO Energy Gap The large value of second-order polarizability, β, which is the measure of the nonlinear optical activity of the molecular system, is associated with the intramolecular charge transfer, resulting from the electron cloud movement through the π-conjugated framework from electron donor to electron acceptor groups. The analysis of the wave function indicates that the electronic absorption corresponds to the transition from the ground to the first excited state and is mainly described by oneelectron excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The atomic orbital compositions of the frontier molecular orbitals of LGP are given in Figure 7. The eigenvalues of HOMO and LUMO and their energy gap reflect the chemical activity of the molecule. The calculated energies of LGP are
HOMO ) -8.3280 eV LUMO ) 0.7028 eV Energy gap ) 9.0309 eV The lowering in the HOMO and LUMO energy gap explains the eventual charge transfer interactions taking place within the molecule and dynamic crystal for the NLO application.
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