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Experimental UV–Visible spectral data for biphenyl-4-carboxaldehyde (B4A), biphenyl-3-carboxaldehyde (B3A), and biphenyl-2-carboxaldehyde (B2A) were obtained in the spectral space 200–400 nm in a solution of CDCl3. In order to substantiate the experimental data, theoretical quantum chemical calculations were made using time-dependent DFT (TD-DFT) formalism employing B3LYP functional in combination with 6–311++G(d,p) for the three molecules in CDCl3 solution phase. Certain global reactivity descriptors were evaluated with the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) (FMOs) analysis, with in the frame work of DFT/B3LYP/6–311++G(d,p) method for the three molecules being investigated. The non-linear optical (NLO) material profiles of the three molecules consisting of their static NLO parameters were theoretically investigated to find their utility for NLO applications. Hyperconjugative interactions, as revealed by natural bond orbital (NBO) analysis, helped to explain NLO behavior of the molecules in terms of intramolecular charge transfer (ICT). The reactive sites of B4A, B3A, and B2A molecules were probed by MESP surface analysis. Experimental and simulated electronic absorption maxima agree well for, B4A, B3A, and B2A. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) orbitals are used to understand π→π* UV–Visible transitions in the three molecules. The molecules are highly reactive and stable. B4A and B3A are good for developing NLO materials. Space around oxygen atom of the aldehyde moiety of the molecules is the most favorable site, for electrophilic attack.

Pyridine-3-carboxylic (or nicotinic) (P3CA) acid has been recorded by using different spectroscopic tools like NMR (13C, 1H), UV–Vis, FT-IR and FT-Raman spectral analysis. The structural conformation examined by potential energy scan for the titled compound was obtained by using DFT computations through B3LYP method with 6–311++G(d,p) basis set. Using optimized monomer structure, the dimeric structures (head-to-tail and tail-to-tail) are obtained. The title compound exhibits inter-molecular as well as intra-molecular hydrogen bonding forming a supramolecular network. The geometrical parameters of both monomer and dimer are coinciding with the observed ones. Using vibrational frequencies, Infrared and Raman intensities were computed. The rms error between the experimental and scaled quantum mechanical frequencies is 9.77 cm−1 for P3CA. From the scaled quantum mechanical method, reliable vibrational assignments were made using PED. The site of chemical reactivity and selectivity of the title compound were estimated from frontier molecular orbitals and local reactivity descriptors. The reactive areas around the compound were evaluated by using MESP. The interactions of donor and acceptor are studied by NBO analysis. The 13C and 1H NMR chemical shifts of the title compound were determined using the gauge independent atomic orbital (GIAO) method in DMSO-d6 solution. By using UV–visible spectrum, the maximum absorption wavelength was obtained and compared with the TD-DFT method.

Fourier transform infrared spectra, for 2-((2-aminopyridin-3-yl) methylene) hydrazinecarbothioamide (APHT) and 2((2-aminopyridin-3-yl) methylene)-N-methylhydrazinecarbothioamide (APMHT), were recorded in the spectral range 4000–400 cm−1. Their Raman counterparts were measured in the spectral region 4000–50 cm−1. Preliminary values of dihedral angles around five rotating bonds C–NH2 pyridyl C–CN, N–N, N–CS and C–NH2 aliphatic in APHT, required for initiating geometry optimization, were obtained by pairing successive bonds and evaluating torsional potential energy for various values of dihedral angle around these bonds in the entire conformational space spanning 0° to 360°. Barrier heights, around five rotating bonds in APHT and six rotating bonds in APMHT were computed, making torsional scans in conformational space from 0° to 360°. This indicated existence of two rotational isomers for APMHT. General valence force field, harmonic vibrational fundamentals, potential energy distribution (PED), along with infrared and Raman intensities were determined using DFT/B3LYP/6-311++G(d,p) level of theory for both the molecules. Good agreement was found between measured and simulated spectra, for APHT and APMHT. This was also true for corresponding Raman spectra. The rms error between experimental and theoretical vibrational frequencies was 9.00 and 6.70 cm−1, for APHT and APMHT, respectively. All vibrational fundamentals were assigned unambiguously, on the basis of computed PED, eigenvectors, and literature range for the first time. These assignments were further supported by comparing with attribution of corresponding bonds in the parent molecule pyridine and a related molecule 2-((2-aminopyridin-3-yl)methylene)-N-ethylhydrazinecarbothioamide, wherever possible.