Rich Collection of n-Propylamine and Isopropylamine Conformers: Rotational Fingerprints and State-of-the-Art Quantum Chemical Investigation

Abstract

The conformational isomerism of isopropylamine and n-propylamine has been investigated by means of an integrated strategy combining high-level quantum-chemical calculations and high-resolution rotational spectroscopy. The equilibrium structures (and thus equilibrium rotational constants) as well as relative energies of all conformers have been computed using the so-called “cheap” composite scheme, which combines the coupled-cluster methodology with second-order Møller–Plesset perturbation theory for extrapolation to the complete basis set. Methods rooted in the density functional theory have been instead employed for computing spectroscopic parameters and for accounting for vibrational effects. Guided by quantum-chemical predictions, the rotational spectra of isopropylamine and n-propylamine have been investigated between 2 and 400 GHz with Fourier transform microwave and frequency-modulation millimeter/submillimeter spectrometers. Spectral assignments confirmed the presence of several conformers with comparable stability and pointed out possible Coriolis resonance effects between some of them.

A Journey from Thermally Tunable Synthesis to Spectroscopy of Phenylmethanimine in Gas Phase and Solution

“…Given the high accuracy of the QC calculations performed (the relevant spectroscopic parameters are collected in Table 1), the computational prediction of the rotational spectrum was expected to match the accuracy [37] required for an unequivocal identification of the E and Z isomers of phenylmethanimine in the gas phase. Indeed, it was straightforward to successfully identify and assign more than one hundred transition frequencies for both PMI isomers.…”

High-Resolution Rotational Spectroscopy and Interstellar Search for Isopropyl Isothiocyanate

“…The resulting PESs suggest that only the trans form has a stable structure (Figure 2). The shape of the trans conformer is shown in Figure 1, while the rotational constants, electric dipole moment components, 14 N nuclear quadrupole coupling constants, planar moment of inertia, and quartic centrifugal distortion constants are given in Table 1 in which the calculated spectroscopic parameters of the conformer of skew− gauche are also given for comparison.…”

“…All of the measured transitions show a hyperfine pattern arising from the 14 N nucleus quadrupole coupling effect. Figure 3 shows a section of the rotational spectrum displaying the rotational assignments and the 14 N quadrupole coupling splitting of the 3 13 ← 2 12 transition.…”

“…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. In addition, rotational spectrum measurements were extended to three singly substituted 13 C, one 15 N, and one 34 S isotopologues in natural abundance.…”

Ab Initio Study of Fine and Hyperfine Interactions in Triplet POH

“…On the other hand, the line-by-line assignment of these complex patterns greatly benefits from the availability of effective theoretical simulations based on accurate quantum-chemical techniques [ 14 , 15 , 17 , 25 ]. In fact, even though computational spectral predictions are always affected by uncertainties in the absolute line position, the hyperfine structures can be reproduced with great accuracy once the critical spectroscopic parameters are computed at a suitable level of theory [ 14 , 25 , 26 ].…”