Infrared spectra and Ab initio computation of molecule Goniofufurone monoacetate (GMA) has been analyzed. The equilibrium geometry, bonding features and harmonic vibrational frequencies of GMA have been investigated with the help of B3LYP density functional theory (DFT) method. Potential energy surface (PES) scan studies have also been carried out by Ab initio calculations using DFT level. The natural bond orbital (NBO) analysis confirms the occurrence of strong intramolecular hydrogen bonding in the molecule.
Keywords: FT-IR; DFT; NBO; PES; Hyper-conjugation
As a part of the continuing programme for the isolation of bioactive principles from Goniothalamus spp., Goniofufurone monoacetate (GMA) is isolated from the leaves of Goniothalamus wynaadensis Bedd.(Annonaceae) [1,2]. The structure of the compound was established with the help of IR, NMR and single crystal X-ray studies [3-5]. During the past decade, the density functional theory (DFT) has emerged as a powerful tool in studying vibrational spectra of fairly large molecules, and is accepted by the Ab initio quantum chemical community as a popular method for the computation of molecular structure, vibrational frequencies and energies of chemical reaction . The present investigation aims to understand the structural and bonding features and molecular information such as electronic delocalization, hyper-conjugation and intramolecular charge transfer of the molecule, GMA using DFT calculations along with FT-IR spectral analysis. The change in electron density (ED) in the σ * antibonding orbitals and E(2) energies has been calculated by natural bond orbital (NBO) analysis. The potential energy surface scan (PES) values and the thermodynamical parameters of the GMA molecule were calculated and analyzed. From the molecular orbital analysis the Highest molecular orbital (HOMO) and Lowest unoccupied molecular orbital (LUMO) energies of the GMA molecules are calculated and analyzed.
The quantum chemical computations of GMA has been performed using Gaussian '03 program package  at the Becke3-Lee-Yang-Parr (B3LYP) level [8,9] with standard 6-31G(d) basis set . Subsequently, the harmonic frequencies and infrared intensities were calculated. The computed optimized geometry and the corresponding parameters are confirmed to be a minimum energy conformation. The computed wavenumbers were scaled by an empirical scaling factor of 0.9613  to fit with the experimental wavenumbers, which accounts for systematic errors caused by basis set incompleteness, neglect of electron correlation and vibrational anharmonicity.
The plant material was collected from the Wynaad hills of Kerala, India during December 2005 and was identified by TBGRI, Palode where a voucher specimen has been deposited. Shade dried leaf powder (500) gm was extracted with ethyl alcohol in a soxhlet. 24 gm of gummy residue was obtained. 20 g of it was chromatographed on a glass column (40 mm×80 cm) using silica gel (100-200 mesh). Elution was done with hexane–ethyl acetate mixtures. 50 mL fractions were collected and were analyzed by TLC in glass plates coated with silica gel. Iodine vapor was used as the detector. The compound was obtained as white needles (m.p.186°C) from 40% ethyl acetate–hexane fractions.
The FT-IR spectrum of GMA was recorded in the region 4000-400 cm-1, with samples in the KBr pellet, using Nicolet Magna 560 FT-IR spectrometer. The resolution of the spectrum is 4 cm-1.
The optimized structure is shown in Figure 1 and the corresponding parameters are given in Table 1. From the optimized geometry the presence of a five-membered lactone ring fused to a furan ring [at the 2,3-position (C15 and C16)] imposes some conformational rigidity on this compound. The values of relevant torsion angles C15-C16-C21- O23= -6.9125°, C15-C21-C22-O23= 2.6337°, indicate that the fivemembered lactone ring adopts the 2E conformation which is significantly deformed (twisted on C15-C16 bond) adopting the shape which is very close to the C-5TC-1 conformation . The value of C12- C13 (1.498 Å), C16-C17 (1.492 Å), C21-C22 (1.468 Å) and C3-C12 (1.523 Å) are larger than the other C-C bonds, which shows the conjugation of electron donor oxygen to the acceptor carbon atom through the double bond carbon. The increase in C15-C21-H32 (132.5°) and C21- C22-O33 (129.8°) bond angles also supports this effect. The variation of C-O bond lengths C13-O14 (1.252 Å), C17-O19 (1.394 Å), C12-O25 (1.395 Å) and C22-O33 (1.366 Å) is due to different environment of oxygen atom in GMA molecule.
|Bond length||Value (Å)||Bond angle||Value (°)||Dihedralangle||Value (°)|
|C5-C6||1.386||C6-C1-H7||120.2||C6 -C1 -C2 -H8||-179.324|
|C1 -H7||1.075||C1-C2-H8||120.2||C2-C3-C4 -H9||179.31|
|C2 -H8||1.074||C3 -C4 -H9||120.1||C3-C4-C5-H10||-179.809|
Table 1: Optimized geometrical parameters of GMA at the DFT level.
|Donor NBO (i)||Acceptor NBO (j)||E(2)a||E(j)-E(i)b||F(i,j)c|
|aEnergy of hyperconjugative interaction (stabilization energy).
bEnergy difference between donor i and acceptor j NBO orb.
cFock matrix element between i and j NBO orbitals.
Table 2: Second-order Perturbation theory analysis of Fock Matrix in NBO basis corresponding to the intramolecular C-H…O hydrogen bonds of GMA.
Potential energy surface (PES)
Theoretical approaches to the link between the structure, energetics and dynamics of disordered materials demand computational techniques that are able to map the low energy portion of the potential energy surface accurately [13,14]. The PES scan for conformational motion of GMA molecule was constructed using energies after the optimization of stationary points geometries found at the DFT level. The calculated result shows that the global minimum energy of GMA was found to be -554437 kcal mol-1. The torsion angle around the phenyl ring (C2-C3-C12-C13) with step angle of 10° is plotted in Figure 2.
The NBO analysis is already proved to be an effective tool for chemical interpretation of hyperconjugative interaction and ED transfer from the filled lone pair orbital. The NBO calculations  were performed using NBO 3.1 program as implemented in the Gaussian 03 package in order to understand various second order interactions between the filled orbitals of one subsystem and vacant orbitals of another subsystem, which is a measure of the intermolecular delocalization or hyperconjugation. NBO analysis provides the most accurate possible ‘natural Lewis structure’ picture of single basis orbital j, because all orbital details are mathematically chosen to include the highest possible percentage of the electron density. The second order Fock matrix was carried out to evaluate the donor–acceptor interactions in the NBO basis . For each donor (i) and acceptor (j), the stabilization energy E(2) associated with the delocalization i→j is estimated as
Where qi is the donor orbital occupancy, εi and εj are diagonal elements and F(i,j) is the off diagonal NBO Fock matrix element.
Second-order perturbation theory analysis of Fock matrix in NBO basis, and the formation of Lewis and Non-Lewis orbitals by the valance hybrids corresponding to the intra-molecular C-H…O hydrogen bonds of GMA is given in Tables 3 and 4. The existence of intra-molecular hydrogen bonds due to the interaction between the lone pair of oxygen and anti-bonding orbitals of LP1O19 – σ*(C4-H9), LP1O27 – σ *(C28-H31), LP2O27–σ *(C28-H29), LP2O27 –σ *(C28-H30) have been confirmed by the NBO result. The strengthening and contraction of the C-H bonds is due to rehybridization , which is revealed by the low value of ED 0.01561, 0.018087, 0.01086 and 0.01572 e in the σ *(C4-H9), σ *(C28-H31), σ *(C28-H29) and σ *(C28- H30) orbitals respectively. The intramolecular C-H…O interactions are also evident from the hydrogen bonding geometry. The intramolecular contacts of O19…H9, O27…H29, O27…H30, and O27…H31 are 2.566, 2.046, 2.045 and 2.122 Å, respectively, which are significantly shorter than the van der Waals separation between the O-H atom (2.73 Å) , indicating the possibility of intramolecular C-H…O interaction.
Table 3: NBO result showing the formation of Lewis and Non-Lewis orbitals by the valance hybrids corresponding to the intramolecular CH… O
|nIR (cm-1)||ncal(cm-1)||IR Intensity||Assignment|
|3537 m||3580||29.3||O19-H2O stretch|
|3094 w||3098||7.4||C-H stretch 2 of Ph ring|
|3065 w||3081||14.5||C-H stretch 20a of Ph ring|
|3007 w||3040||16.7||CH3 asym. stretch|
|2947m||2957||51.8||CH3 sym. stretch|
|2920 w||2918||10.2||C12-H24 stretch|
|2884 w||2861||17||C17-H18 stretch|
|1770 s||1807||927||C22=O33 stretch|
|1605 w||1594||4.3||C-C stretch 8a of Ph ring|
|1588 w||1582||2.3||C-C stretch 8b of Ph ring+C12-H24 ipb|
|1490 w||1484||13.7||C-C stretch 19a of Ph ring|
|1459 w||1459||3.8||CH3 asym. bend|
|1418 w||1437||13.8||CH3 umbrella mode|
|1394 w||1411||2.9||C21-C16-C15 stretch+C13-O14 stretch|
|1379 m||1350||66.3||C12-H24, C17-H18, C21-H32ipb+O19-H20 bend+C-H ipb 3 of Ph ring+CH3 umbrella|
|1342 w sh||1337||27.4||C12-H24 ipb+O19-H20 bend+C-H ipb 3 of Ph ring|
|1329 w||1321||14.8||C-H ipb 3 of Ph ring+O19-H20ipb|
|1310 w||1311||18.9||C12-H24, C17-H18ipb+O19-H20ipb+C-C stretch 14 of Ph ring|
|1305 w||1309||28.8||C12-H24, C17-H18ipb+O19-H20ipb+C-C stretch 14 of Ph ring|
|1283 w||1294||297.4||C12-H24ipb +C28-H31ipb+O19-H20ipb+C-C stretch 14 of Ph ring|
|1256 m sh||1253||30.6||C12-H24, C17-H18, C21-H32 ipb+O19-H20 bend+C-C stretch 14 of Ph ring|
|1238 m||1226||19||C12-H24, C17-H18, C21-H32ipb+O19-H20 bend+C-H ipb 3 of Ph ring|
|1194 m||1171||9||C12-H24, C17-H18, C21-H32ipb+O19-H20 bend|
|1153 m||1155||469.8||C-H ipb 18a of ph ring|
|1142 w||1147||2||CH3 opb|
|1105 m||1104||107.4||C12-H24, C17-H18, C21-H32ipb+O19-H20 bend|
|1081 msh||1081||117.8||C-H ipb 15 of Ph ring+C12-H24 ipb|
|1069 m||1064||28.4||C-H ipb 15 of Ph ring+C12-H24 ipb|
|1041 m||1058||68.4||C17-H18, C21-H32ipb|
|1027 w||1018||4.7||C-C radial skeletal 12 of Ph ring|
|998 w||999||40.6||C12-H24, C17-H18 ipb|
|985 w||985||181.4||C12-O25,C28-O27 stretch|
|921 vw||940||7.3||C-H opb 17a of Ph ring|
|791 w||781||71.8||C-H opb 10b of Ph ring|
|751 w||759||29.5||C-H opb 10b of Ph ring|
|698 m||698||59.1||C-H opb 11of Ph ring+OH opb|
|623 w||609||3.5||OpSkeletal 6bof Ph ring|
|604 vw||603||45||OpSkeletal 6bof Ph ring|
|548 m||578||147.1||C12-H24, C17-H18 opb|
|535 m||528||91.2||OpSkeletal 6bof Ph ring +C13-O14-C15 bend|
|419 w||436||93.5||C12-O25-C26, C26-O27-C28, C16-O22-C24 bend|
|407 w||411||20.7||C12-O25-C26,C26-O27-C28 bend|
Table 4: FT-IR and calculated vibrational wavenumbers, infrared intensities and assignment for GMA.
Vibrational spectral analysis
The analysis of the vibrational spectra is based on the FT-IR spectrum and the vibrational wavenumbers computed at the B3LYP/ 6-31G(d) level with their scaled value. The calculated vibrational wavenumbers, measured infrared band positions and their tentative assignments are presented in Table 4. The observed and simulated FTIR spectra of GMA are given in Figure 3. The vibrational analysis was performed on the basis of the characteristic vibrations of hydroxyl, carbonyl, phenyl, and methyl group.
Hydroxyl vibration: The hydroxyl groups are likely to be the most sensitive to the environment and so they show pronounced shifts in the spectrum of the hydrogen bonded species. The hydroxyl group absorbs strongly in the 3700-3584 cm-1 region, whereas the existence of intermolecular hydrogen bond formation can lower the O-H stretching wavenumber to 3550-3200 cm-1 region [19,20]. The IR spectrum in the high region shows a broad intense band at 3537 cm-1attributed to hydrogen-bonded OH stretching. The in-plane OH deformation vibration usually appears as a strong band in the region 1440-1260 cm-1 . The medium band at 1379 cm-1 corresponds to the in-plane bending mode of the hydroxyl vibration. The OH out of plane deformation vibration lies in the 290-320 cm-1 for free OH and in the region 517-710 cm-1 for associated OH . The medium band at 698 cm-1 corresponds to out-of-plane bending vibration of hydroxyl group.
Carbonyl vibration: Carbonyl group vibrations in ketones are the best characteristic bands in the vibrational spectra, and for this reason, such bands have been the subject of extensive studies . The carbonyl stretching vibrations of ketones have normally strong intensity and are expected in the region 1715-1680 cm-1 . In GMA the carbonyl vibration C22=O33 stretch is calculated as 1807 cm-1and is observed at 1770 cm-1 as a strong intensity. This higher wavenumber 1770 cm-1 indicates the stretching vibration of C=O bond is not interacting with other atoms in the vicinity of the ring. The C13-O14 stretch is calculated as 1411 cm-1and is observed at 1394 cm-1 as a weak intensity. Normally the C-O-C bending vibration is expected in the region around 440 cm-1. In GMA the C12-O25-C26, C26-O27-C28, C16-O22-C24 bending is observed as a weak band at 419 cm-1.
Phenyl ring vibration: The various normal vibrations of the monosubstituted benzene ring are assigned according to Wilson’s numbering convention . Of the five allowed C-H stretching modes 2, 20a, 20b, 7a and 7b for the mono substituted ring, 2 and 20a are found to be active. The C-H stretching vibration appears to be very weak, which is due to the charge carrier localization in the phenyl ring . The weak bands observed at 3094 and 3065 cm-1 are assigned for mode 2, 20a, respectively. Normal vibrations 8a, 8b, 19a, 19b and 14 are classified as the C-C stretching vibrations of the monosubstituted rings. The normal mode 8a has been observed at 1605 cm-1 as a weak intensity and is calculated at 1594 cm-1. The weak band observed at 1588 cm-1 in IR is assigned to 8b mode. The 19a mode in monosubstituted ring can be expected near 1500 cm-1 as a stronger band and 19b appear as a weak band around 1470 cm-1 . The weak band observed at 1490 cm-1in is due to contribution of 19a mode. The C-C stretching mode 14 is observed as a medium shoulder band at 1256 cm-1 which is attributed to the isotopic effect of the carbon.
Normal vibration 3, 9a, 15, 18a and 18b are classified as C-H inplane bending vibrations of monosubstituted phenyl rings. Mode 3 is coupled with O-H in-plane bending, methane in-plane bending and methyl deformation, which are observed as a series of bands at 1379, 1342, 1329 cm-1. The higher intensity of the IR band at 1379 cm-1 is due to the contribution of the methyl umbrella mode . The intense band at 1153 cm-1 is assigned to the mode 18a and is calculated to be 1155 cm-1. The C-H in-plane bending mode 15 is observed at 1081 and 1069 cm-1 as a medium intensity which is coupled with methane bending.
The absorption bands arising from the C-H out-of-plane vibrations are usually observed in the region 1000-675 cm-1 . Although the allowed C-H out of plane modes are 5/10b, 11/17b, 10a and 17 a, only the modes 17a, 10b and 11 are active for GMA. The mode 17a is observed in IR at 921 cm-1 as a very weak intensity. The weak intense bands in IR at 751 and 698 cm-1 are assigned for C-H out of plane bending modes 10 and 11. The in-plane ring deformations derived from the b1u benzene vibration 12 gives rise to an intense band in monosubstituted benzene  at 1010-990 cm-1. The radial skeletal vibration 12 of GMA is observed as weak band at 1027 cm-1 and the ring deformation 6b is observed as a weak band at 604 cm-1.
Methyl group vibration: The asymmetric C-H stretching mode of CH3 is expected around the region 2980 cm-1 and the symmetric one is expected in the region 2870 cm-1[19,30]. The CH3 asymmetric stretching vibration appears as a weak band at 3007 cm-1. The symmetric stretching mode is observed as a medium intense band at 2947 cm-1. The shifting of higher wavenumber in the IR spectrum is due to the influence of electronic effect resulting from hyperconjugation and induction of methyl group with the aromatic ring system. The asymmetric and symmetric bending vibrations of methyl groups normally appears in the region 1465-1440 cm-1 and 1390-1370 cm-1, respectively, [21,28]. The weak band at 1418 cm-1 in the infrared spectrum is attributed to the CH3 asymmetric bending, and the weak band at 1418 cm-1 is assigned to the CH3 symmetric bending mode.
Thermo dynamical parameters
Thermodynamic properties, such as heat of formation (HOF), enthalpy and entropy, are important parameters to evaluate the explosive properties of an energetic compound. For stable compounds, of course, there are many tables that contain experimental data. However, for energetic materials and unstable compounds, determination of the thermodynamic properties is impractical or dangerous. At the present time one may calculate them from empirical models such as group additive methods. Such methods work very well for hydrocarbons, but often fail for other classes of compounds . The HF and DFT levels calculated thermodynamic parameters are presented in Table 5. Scale factors have been recommended  for an accurate prediction in determining the zero-point vibration energies (ZPVE) and the entropy S. The variations in the ZPVE s seem to be insignificant. The total energies are found to decrease with the increase of the basis set dimension. The changes in the total entropy of BMP at room temperature at different basis sets are only marginal.
Table 5: Theoretically computed total energies (kJ mol−1) zero-point vibrational energies (kcal mol−1), rotational constants (GHz), entropies (cal mol−1 K−1) and dipolemoments (debye) for GMA.
One property of the electronic structure that changes in a consistent manner for a wide variety of chemical reactions is the difference in energies between the highest occupied molecular orbital (HOMO) and the LUMO, the HOMO–LUMO gap. For many reactions involving the interaction of these two orbitals the value of the HOMO–LUMO gap is a minimum at, or near, the transition state .The electronic structures of Goniofufurone monoacetate, have been analyzed with B3LYP/6-31 G(d) level. The delocalized molecular structure HOMO, HOMO-1, HOMO-2 and LUMO, LUMO+1, LUMO+2 of GMA are plotted in Figure 4. From the figure HOMO of GMA is delocalized on π orbitals of the lactone ring. The reported HOMOs have a bonding π character, that the HOMO and HOMO-2 show a partial non-bonding character due to the carboxylic oxygen lone pair. The LUMOs have antibonding π* character and are highly delocalized over the whole molecules, all the LUMOs show more charge delocalization on the benzene ring.
The understanding of the structure–property correlation of Goniofufurone monoacetate (GMA) materials is important for the development of pharmacological drug materials. The structural features of the highly efficient pharmacological drug GMA as well as the vibrational spectral investigations have been carried out from FTIR spectra aided by the DFT computations. The vibrational analysis of GMA was performed using DFT calculations. From the optimized geometry the torsion angles C15-C16-C21-O23 = 6.9125°, C15-C21-C22- O23 = 2.6337° show that the five-membered lactone ring adopts the 2E conformation. The blue shifting of CH3 symmetric stretching mode in the IR spectrum is due to influence of electronic effect resulting from hyperconjugation and induction of methyl group with the aromatic ring system. NBO analysis confirms the C-H...O intramolecular hydrogen bonding. The calculated HOMO-LUMO energy was comparatively low and it substantiates the possibility of the intramolecular charge delocalization. These charge delocalization stimulate the pharmaceutical property in the molecule.
Authors B. R. K, M. D. A and G. J are thankful to KSCSTE, Govt. of Kerala for financial assistance.