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

Abstract

The pure rotational spectrum of allyl isothiocyanate (CH2=CHCH2-NCS) was collected from 4-26 GHz using Fourier transform microwave (FTMW) spectroscopy. Its analysis revealed the presence of two conformers that arise due to variation in the CCCN and CCNC dihedral angles.The observed spectrum is consistent with the accompanying potential energy surfaces derived using quantum chemical calculations at the B3LYP-D3(BJ) and MP2 levels of theory. Together, this experimental and theoretical study unequivocally identifies a new conformer (I) as the global minimum geometry. The spectral assignment of this new conformer is verified by the observation of transitions consistent with its 34 S, 13 C and 15 N isotopologues and with the characteristic 14 N quadrupole hyperfine patterns. For conformer I, the substitution (rs) and effective ground state (r0) structures were derived and reveal contributions from a large amplitude motion in the CCNC angle.The remaining geometric parameters compare well with the equilibrium structure (re) from B3LYP-D3(BJ)/cc-pVQZ calculations. The derived CNC bond angle of 152.6(3) o for conformer I of allyl-NCS is found to be ~15 o larger than that of allyl-NCO (137.5(4) o ), which is in line with a change in the hybridization at nitrogen from an orbital with more ~sp character in allyl-NCS to ~sp 1.5 in allyl-NCO as determined via natural bond orbital analyses.

Structure, Stability, and Spectroscopic Properties of Small Acetonitrile Cation Clusters

“…In order to explore a lower limit for the errors in the spectrum simulations, the experimental spectroscopic constants were employed to simulate an empirical spectrum. For low J values ( J max = 10), the average frequency displacement was around 0.012 MHz, in accordance with previous works where this empirical approach was used (e.g., refs and ). Moreover, following the strategy described in the work of Puzzarini, we computed empirical parameters for CH 3 CN, scaled by using the experimental/theoretical ratio obtained with both measured and calculated values of the rotational constant B .…”

High-Resolution Rotational Spectroscopy and Interstellar Search for Isopropyl Isothiocyanate

“…Laboratory investigations of the rotational spectra of several RNCS molecules have been reported for methyl isothiocyanate, 21 phenyl isothiocyanate, 22,23 ethynyl isothiocyanate, 24 and allyl isothiocyanate. 25 In the present work, the pure rotational spectrum of isopropyl isothiocyanate ((CH 3 been reported by Durig et al, 26 where only the value of the sum of (B + C) rotational constants was roughly estimated. 26 The pure rotational spectrum was remeasured at higher instrumental resolution and higher accuracy, and 14 N nuclear quadrupole hyperfine structure splittings were well resolved.…”

“…Indeed, HSCN and HNCS have been identified in the chemically rich Sagittarius B2 region (Sgr B2). Laboratory investigations of the rotational spectra of several RNCS molecules have been reported for methyl isothiocyanate, phenyl isothiocyanate, , ethynyl isothiocyanate, and allyl isothiocyanate . In the present work, the pure rotational spectrum of isopropyl isothiocyanate ((CH 3 ) 2 CHNCS), a branched NCS-bearing molecule, was measured in the frequency region of 2–20 GHz using a high-resolution Fourier transform microwave (FTMW) spectrometer.…”

Targeting the Rich Conformational Landscape of N‐Allylmethylamine Using Rotational Spectroscopy and Quantum Mechanical Calculations

“…Allyl derivatives of the type H 2 C=CHÀ CH 2 À X tend to show a diverse conformational behavior which arises from internal rotations around its two single bonds. [12][13][14] Depending on the nature of the X substituent, a variety of effects, including steric hindrance, London dispersive forces and hydrogen bonding, can rule their conformational preferences. One example is the many theoretical [14] and experimental [15][16][17][18][19] investigations required to partially understand the relative energy ordering of the four observed conformers of allylamine (H 2 C=CHÀ CH 2 À NH 2 ).…”

“…Allyl derivatives of the type H 2 C=CH−CH 2 −X tend to show a diverse conformational behavior which arises from internal rotations around its two single bonds [12–14] . Depending on the nature of the X substituent, a variety of effects, including steric hindrance, London dispersive forces and hydrogen bonding, can rule their conformational preferences.…”