Yordanka Dimitrova

Theoretical study of hydrogen-bonded formaldehyde complexes

The hydrogen-bonded complexes involving formaldehyde and a series of proton donors of varying strengths, have been investigated at different levels of ab initio MO theory. The structures of the studied complexes were SCF optimized at the 6-31G basis set level. The binding energy was estimated employing basis set superposition correction, zero-point vibrations and MP2 correlation contribution at the different basis set: STO-3G; 6-31G; MP2/6-31G; 6-31G**; MP2/6-31G**; 6-311G(2d, 2p) and MP2/6-311G(2d, 2p). Linear relationships were found of the calculated binding energy with: the calculated shift in the carbonyl stretching frequency, the changes in carbonyl bond length and the optimum value of hydrogen-bond distance; furthermore the calculations confirm a parallel trend between the proton-donor ability and the strength of the hydrogen bond.

Theoretical study of structures and stabilities of hydrogen-bonded phenol–water complexes

Abstract The hydrogen-bonded complexes of phenol with 1–4 water molecules are investigated at different levels of ab initio molecular orbital (MO) theory. The structures of the studied complexes have been SCF and MP2 optimized with different basis sets. The most stable structures of the phenol–water complexes have been determined. The dissociation energy has been estimated employing basis-set superposition correction, zero-point vibrations and MP2 correlation contribution to the dissociation energy at different basis sets of ab initio calculations. The Mulliken population analysis has been used to investigate the charge distribution in the hydrogen-bonded complexes of phenol with 1–4 water molecules. The changes in the atomic charges upon hydrogen bonding for the complexes studied have been determined.

Solvent effects on vibrational spectrum of hydrogen-bonded complex PhOH⋯H2O. An ab initio study

Abstract The vibrational features characterising the hydrogen-bonded interaction between PhOH and H 2 O have been studied. The vibrational spectra for free and complexed PhOH and H 2 O have been predicted by ab initio calculations at different levels: 3–21G/SCF, 6–31G/SCF and 6-31G/MP2. The changes in the vibrational characteristics (vibrational frequencies, infrared intensities and Raman activities) upon hydrogen bonding have been estimated. It was established that the most sensitive to the complexation is the stretching O–H vibration of the phenol site. In agreement with the experiment, its vibrational frequency is shifted to lower wavenumbers. The magnitude of the wavenumber shift is indicative of a relatively strong OH⋯H hydrogen-bonded interaction. The ab initio calculations at different levels predict an increase of the IR intensity up to 50 times and of the Raman activity up to four times. The remaining vibrations (stretching, bending and torsion) are less sensitive to the hydrogen bonding. Their vibrational characteristics are changed to a less extent.

Solvent effects on vibrational spectra of hydrogen-bonded complexes of formaldehyde and water: an ab initio study

Abstract The vibrational spectrum of the system H 2 CO + H 2 O has been predicted by ab initio calculations employing the 6-31G ∗∗ basis set. The changes in the vibrational characteristics (vibrational frequencies, infrared intensities and Raman activities) from free monomer to a complex have been evaluated. In agreement with experiment, the ab initio calculations show that the stretching C  O and OH vibrations involved in hydrogen bonding are shifted to lower frequency, corresponding to bond weakening. It was established that for the asymmetric H 2 CO…H 2 O structure the ab initio solvent shifts show the best agreement with the experimentally measured shifts.

Ab initio study of structures of hydrogen-bonded nitric acid complexes

Abstract The structures of the hydrogen-bonded complexes of nitric acid with various bases: CH 4 ⋯ H – ONO 2 , N 2 ⋯H–ONO 2 , OC⋯H–ONO 2 , NO 2 ⋯H–ONO 2 , (HNO3)2 and H 3 N⋯H–ONO 2 are studied extensively by ab initio calculations at SCF and MP2 levels with various basis sets: 6-31G/SCF, 6-31G/MP2, 6-31G**/SCF, 6-31G**/MP2, 6-31+G**/SCF, 6-31+G**/MP2, 6-311+G**/SCF and 6-311+G**/MP2 and density functional B3LYP/6-31G(d,p) calculations. Full geometry optimization was made for the complexes studied. After the consideration of the strength of the hydrogen bond, the studied hydrogen-bonded complexes can be order as follows: H 3 N⋯H–ONO 2 , (HNO 3 ) 2 ,NO 2 ⋯H–ONO 2 ,OC⋯H–ONO 2 ,N 2 ⋯H–ONO 2 ,CH 4 ⋯H–ONO 2 .

Structure, stability and vibrational spectrum of the hydrogen-bonded complex between HNO3 and H2O: Ab initio and DFT studies

The structure, stability and vibrational spectrum of the binary complex between HONO2 and H2O have been investigated using ab initio calculations at SCF and MP2 levels with different basis sets and B3LYP/6-31G(d,p) calculations. Full geometry optimization was made for the complex studied. It was established that the hydrogen-bonded H2O...HONO2 complex has a planar structure. The corrected values of the dissociation energy at the SCF and MP2 levels and B3LYP calculations are indicative of relatively strong OH...O hydrogen-bonded interaction. The changes in the vibrational characteristics (vibrational frequencies and infrared intensities) arising from the hydrogen bonding between HONO2 and H2O have been estimated by using the ab initio calculations at SCF and MP2 levels and B3LYP/6-31G(d,p) calculations. It was established that the most sensitive to the complexation is the stretching O-H vibration from HONO2. In agreement with the experiment, its vibrational frequency in the complex is shifted to lower wavenumbers. The predicted frequency shift with the B3LYP/6-31G(d,p) calculations (-439 cm(-1)) is in the best agreement with the experimentally measured (-498 cm(-1)). The intensity of this vibration increases dramatically upon hydrogen bonding. The ab initio calculations at the SCF level predict an increase up to five times; at the MP2 level up to 10 times and the B3LYP/6-31G(d,p) predicted increase is up to 17 times. The good agreement between the predicted values of the frequency shifts and those experimentally observed show that the structure of the hydrogen-bonded complex H2O...HONO2 is reliable.

Theoretical study of the stability of hydrogen-bonded complexes of nitric acid with various bases

Abstract The stability of the hydrogen-bonded complexes of nitric acid with various bases: CH 4 ⋯H–ONO 2 , N 2 ⋯H–ONO 2 , OC⋯H–ONO 2 , CO⋯H–ONO 2 , NO 2 ⋯H–ONO 2 , (HNO 3 ) 2 and H 3 N⋯H–ONO 2 , has been studied extensively by ab initio calculations at the SCF and MP2 levels with various basis sets: SCF/6-31G(d,p), MP2/6-31G(d,p), SCF/6-31+G(d,p), MP2/6-31+G(d,p), SCF/6-311+G(d,p) and MP2/6-311+G(d,p), and density functional B3LYP/6-31G(d,p) calculations. The dissociation energy has been estimated by employing the basis set superposition error correction, zero-point vibrations and MP2 correlation contribution to the dissociation energy. The studied hydrogen-bonded nitric acid complexes in decreasing order of stability are as follows: H 3 N–H–ONO 2 >(HNO 3 ) 2 >NO 2 ⋯H–ONO 2 >OC⋯H–ONO 2 >CO⋯H–ONO 2 >N 2 ⋯H–ONO 2 >CH 4 ⋯H–ONO 2 .

Vibrational spectra of benzonitrile and its radical anion: an ab initio study

Abstract The vibrational spectra of benzonitrile and its radical anion have been predicted by ab initio calculations at the 6-31G and 6-31G∗∗ levels. The changes in the vibrational characteristics accompanying the conversion of the neutral molecule of benzonitrile into the corresponding radical anion have been estimated. The ab initio calculations predict a decrease in the frequencies of the stretching and deformation vibrations in the radical anion and in most cases an increase in the intensities. It was established that the radicalization leads to the most significant changes in the vibrational characteristics of the C  N stretching vibration: a substantial decrease in the frequency νCN and an unusual increase in the intensity of the νCN vibration.

Vibrational frequencies and infrared intensities of acetonitrile coordinated with metal cations: an ab initio study

Vibrational frequencies and infrared intensities have been calculated at the 6-31G and 6-31G∗∗ levels for acetonitrile and for the complexes of acetonitrile with Li+ and Na+ cations. The changes in the infrared characteristics from an isolated acetonitrile to acetonitrile coordinated with metal cations (Li+ and Na+) have been evaluated. The ab initio calculations predict an essential increase of the intensities of the stretching CN, CC and deformation CH3, CCN vibrations in the complexes of acetonitrile with Li+ and Na+ cations.

Ab initio study of the vibrational spectra of the hydrogen-bonded nitric acid dimer.

The changes in the vibrational characteristics characterizing the dimerization of nitric acid have been investigated by ab initio calculations at the MP2 level, with 6-31G(d,p) and 6-31 + G(d,p) basis sets, and B3LYP/6-31G(d,p) calculations. The most consistent agreement between the computed values of the frequency shifts for the planar fully symmetric structure (2A) and those experimentally observed suggests that this structure is preferred. It was established that the most sensitive to the complexation is the stretching O-H vibration. The values of the frequency shift (-306 cm(-1)) is indicative for the formation of the relatively strong hydrogen bonds. The calculations predict an increase of the infrared intensity of the stretching O-H vibration in the nitric acid dimer more than 26 times.