M. Ramalingam

Molecular structure and vibrational spectra of 3-amino-5-hydroxypyrazole by density functional method.

In this work, we report a combined experimental and theoretical study on molecular and vibrational structure of 3-amino-5-hydroxypyrazole (3A5HP). The Fourier transform infrared and Fourier transform Raman spectra of 3A5HP were recorded in the solid phase. The molecular geometry and vibrational frequencies of 3A5HP in the ground state have been calculated by using the density functional method B3LYP with basis sets, 6-311++G(d,p), 6-311+G(3df,2p), 6-311+G(3df,2pd), CC-pVDZ, aug-CC-pVDZ and CC-pVTZ. The optimized geometrical parameters obtained by B3LYP show best agreement with the experimental values. The theoretical spectrograms for FT-IR and FT-Raman spectra of the title molecule have been constructed.

Vibrational spectroscopic studies and DFT calculations of 4-aminoantipyrine.

The pyrazole derivative, 4-aminoantipyrine (4AAP), used as an intermediate for the synthesis of pharmaceuticals especially antipyretic and analgesic drugs has been analyzed experimentally and theoretically for its vibrational frequencies. The FTIR and FT Raman spectra of the title compound have been compared with the theoretically computed frequencies invoking the standard 6-311g(d,p) and cc-pVDZ basis sets at DFT level of theory (B3LYP). The harmonic vibrational frequencies at B3LYP/cc-pVDZ after appropriate scaling method seem to coincide satisfactorily with the experimental observations rather than B3LYP/6-311g(d,p) results. The theoretical spectrograms for FT-IR and FT-Raman spectra of 4AAP have been also constructed and compared with the experimental spectra. Additionally, thermodynamic data have also been calculated and discussed.

Harmonic analysis of vibrations of morpholine-4-ylmethylthiourea: A DFT, midinfrared and Raman spectral study

The FTIR and FT-Raman spectra of morpholine-4-ylmethylthiourea (MMTU) were recorded in the region of mid-IR (400-4,000 cm(-1)). Initial geometry generated from the standard geometrical parameters was relaxed without any constraint on the potential energy surface at MP2 and DFT levels adopting the standard 6-31++G and 6-311+G basis set. With the help of two specific scaling procedures the computed harmonic frequencies have been compared with the observed vibrational wave numbers of FTIR and FT-Raman spectra and assigned to different normal modes of the molecule. Most of the vibrational modes have wave numbers in the expected range. The appropriate theoretical spectrograms of the IR spectra of MMTU have been also constructed.

SINGLET METHYLENE AND HALOCARBENES INSERTIONS INTO POLAR N–H BONDS OF AMINES

The insertion of singlet 1CH2, 1CHF, and 1CF2 into polar N–H bonds of RNH2 (R = –CH3, –C2H5, –C3H7 or –CH(CH3)2) has been investigated at HF, MP2, and DFT levels using 6-31g (d, p) basis set. The insertions proceed via a two-step mechanism. The potential energy surface exploration identifies an ylide formation followed by the 1,2-proton shift concertedly giving the secondary amine. The barrier height varies with the type of carbene and amine. HOMO of amines and LUMO of carbenes control the initial interactions and it is confirmed by NBO charge analysis.

Ab Initio and DFT Investigations of the Mechanistic Pathway of Singlet Bromocarbenes Insertion into C-H Bonds of Methane and Ethane

The mechanistic pathway of singlet bromocarbenes (1CHBr and 1CBr 2 ) insertions into the C-H bonds of methane and ethane have been analysed at ab initio (HF and MP2) and DFT (B3LYP) level of theories using 6-31g (d, p) basis set. The QCISD//MP2/6-31g(d, p) level predicts higher activation barriers. NPA, Mulliken and ESP charge analyses have been carried out along the minimal reaction path by the IRC method at B3LYP and MP2 levels for these reactions. The occurrence of the TSs either in the electrophilic or nucleophilic phase and net charge flow from alkane to carbene in the TS has been identified through NBO analyses.

C-H functionalisation through singlet chlorocarbenes insertions – MP2 and DFT investigations

The insertion reactions of singlet mono and dichlorocarbenes (1CHCl and 1CCl2) into primary, secondary and tertiary C-H bonds of methane, ethane, propane, n-butane and iso-butane have been investigated at ab initio (MP2) and DFT levels invoking 6-31g(d, p) basis set. Among the σ and π insertions into the said alkane C-H, both MP2 and DFT predict that the σ approach is slightly favoured over the π approach. For inverted σ approach the barrier heights have been found to be ca. 2 to 5 times that of the normal σ approach.