Centrifugal distortion analysis of the microwave spectrum of allyl isothiocyanate (C-gauche conformer)

Abstract

The ground state microwave rotational spectrum of the C-gauche conformer of allyl isothiocyanate has been reinvestigated by extending the frequency range up to 37.0 GHz. A detailed centrifugal distortion analysis has been carried out with previously reported and newly assigned microwave rotational transition frequencies. The evaluated rotational and centrifugal distortion constants are presented.

Conformers of Allyl Isothiocyanate: A Combined Microwave Spectroscopy and Computational Study

“…37 Similar results were observed in the benchmark study of the PES of the isovalent allyl-NCO compound, where among more than 20 functionals tested, dispersion uncorrected DFT methods failed to describe the proper conformational distribution. 11 Thus, although allyl-NCS was investigated previously using both computational 16,38 and experimental 14,15,16 methods, the present work is the first study to predict and confirm the presence of conformer I, the lowest energy conformation.…”

“…The fact that conformer II was not observed in a static gas sample 14,15 certainly calls into question whether this is a stable conformer.…”

“…11 Such a flat PES along the δ = 0 coordinate calls into question whether the ZPE corrections, based on harmonic vibrations, are valid, but nevertheless, conformer II was a true minimum on the PES of allyl−NCS tested at the B3LYP-D3(BJ) (cc-pVQZ, aug-cc-pVTZ, aug-cc-pVQZ) and MP2 (cc-pVQZ, aug-cc-pVTZ) levels based on the absence of imaginary frequencies. The fact that conformer II was not observed in a static gas sample 14,15 certainly calls into question whether this is a stable conformer.…”

“…Initially, a broadband spectrum was recorded from 8 to 18 GHz in segments of 2 GHz using the cp-FTMW spectrometer. Rotational transitions belonging to the parent species of two conformers, later determined to be conformers I and III (see Computational Methods below), were assigned in addition to the rotational transitions of their minor heavy isotopologues ( 13 C, 15 N, and 34 S) in natural abundance. A 3 GHz portion of the cp-FTMW spectrum is shown in Figure 2.…”

“…From previous spectroscopic reports involving allyl−NCS, the conformational story appears to mimic that found in the earlier studies of its oxygen analogue. Using Stark-modulated microwave spectroscopy, 14,15 a gauche conformer was reported for allyl−NCS, while a purported lower-energy cis conformer based on 13 C NMR experiments 16 was not observed. Based on this, it is clear that the conformational space of allyl−NCS requires further study to explore whether a new, dispersion-stabilized conformer is present and how the change from oxygen to sulfur influences the conformational equilibrium.…”