Bond lengths and band strengths of non-equivalent C-H bonds of 2-methyl-1,3-butadiene using density functional theory, Fourier transform IR, and cavity ringdown techniques

The C-H fundamental and overtone spectra (Delta upsilon = 1- 4) of non-equivalent C-H bonds of isoprene (CH2=C(CH3)-CH=CH2) have been obtained using a Fourier transform infrared spectrometer. The C-H vibrational overtone spectra (Delta upsilon = 5, 6) have been recorded using phase shift (PS) and exponential decay cavity ring down (CRD) techniques, respectively at 293 K. Theoretical density functional theory (DFT) calculations of bond lengths, harmonic, and anharmonic frequencies of isoprene and partially deuterated isoprene molecules, as well as computer deconvolution of the bands are used to identify transitions corresponding to non-equivalent C-H bonds. Harmonic frequencies omega(e) and anharmonicities omega(e)x(e) are reported for the C-H (a) in plane and C-H (b) out of plane bonds of the methyl group, the single olefinic nonterminal C-H (o), and the four olefinic terminal C-H bonds that are defined in two groups as C-H (cis) and C-H (trans). From the Delta upsilon = 6 peak wavenumbers, correlation functions are used to calculate C-H bond lengths, isolated frequencies, and dissociation energies. Band strengths, oscillator strengths, and transition moments are obtained experimentally for all transitions. The significance of the assigned vibrational transitions and integrated intensities of isoprene for research on Earth's atmosphere and as a potential indicator of life in exoplanets with oxygen-deprived atmospheres is examined.