Pyridinecarboxaldehydes: Structures, vibrational assignments and molecular characteristics using experimental and theoretical methods

The main focus of this article is on the structural, energetic and vibrational properties of three monosubstituted pyridines, wherein one of the hydrogen atoms of the pyridine is replaced by an aldehyde moiety. To this end, we recorded the Fourier transform infrared, Fourier transform Raman and UV–visible spectra of picolinaldehyde (PA), nicotinaldehyde (NA) and isonicotinaldehyde (IA) at 4000–450 cm^-1, 4000–50 cm^-1 and 200–400 nm, respectively. The initial value of the torsional angle around the C–C_α bond, needed for initiating geometry optimisation, was determined by calculating torsional potential energy for various values of dihedral angle around this bond in the entire conformational space from 0 deg to 360 deg for the three molecules. Quantum chemical calculations were made at the DFT/B3LYP/6–311 +  + G(d,p) level of theory for PA, NA and IA to determine structure parameters in the ground state (in the gas phase), barrier height around the C–C_α bond, the general valence force field, harmonic vibrational fundamentals, potential energy distribution (PED) and infrared and Raman intensities. A time-dependent version of density functional theory (TD-DFT) was employed to evaluate oscillator strengths and absorption maxima (λ_max) in CDCl₃ solution for electronic transitions. Structure parameters, IR, Raman and UV–Vis spectra exhibited good agreement between the theoretical and experimental parameters. Complete vibrational assignments were made for the three molecules unambiguously, using PED and eigenvectors calculated in the process, for the first time. The rms error between observed and simulated vibrational frequencies was 9.9, 10.4 and 9.4 cm^-1, for PA, NA and IA, respectively, on scaling. In addition, we made a theoretical evaluation of nonlinear optical (NLO) properties, frontier molecular orbital (FMO) parameters, natural bond orbital (NBO) characteristics, and molecular electrostatic potential (MESP) surface analysis, along with natural population analysis (NPA) studies, in order to make the characterisation of the molecules under investigation as complete as possible. The dimeric structures of these molecules caused by the formation of intermolecular hydrogen bonds were computed at the same level of theory as used for their corresponding monomers.(author)