Halogen bonding properties of 4-iodopyrazole and 4-bromopyrazole explored by rotational spectroscopy and ab initio calculations

Abstract

The combination of halogen- and hydrogen-bonding capabilities possessed by 4-bromopyrazole and 4-iodopyrazole has led to them being described as "magic bullets" for biochemical structure determination. Laser vaporisation was used to introduce each of these 4-halopyrazoles into an argon gas sample undergoing supersonic expansion prior to the recording of the rotational spectra of these molecules by chirped-pulse Fourier transform microwave spectroscopy. Data were obtained for four isotopologues of 4-bromopyrazole and two isotopologues of 4-iodopyrazole. Isotopic substitutions were achieved at the hydrogens attached to the pyrrolic nitrogen atoms of both 4-halopyrazoles and at the bromine atom of 4-bromopyrazole. The experimentally determined nuclear quadrupole coupling constants, χ(X) and χ(X)-χ(X), of the halogen atoms (where X is the halogen atom) of each molecule are compared with the results of the ab initio calculations and those for a range of other halogen-containing molecules. It is concluded that each of 4-bromopyrazole and 4-iodopyrazole will form halogen bonds that are broadly comparable in strength to those formed by CHX and CFX.

Conformational isomers oftrans-urocanic acid observed by rotational spectroscopy

“…A detailed description of the laser ablation source and chirpedpulse Fourier transform microwave (CP-FTMW) spectrometer used for the experiments has been published previously. [15][16][17][18] Molecules within the expanding gas sample are polarised by a chirped microwave pulse of 1 μs in duration. The 2.0 -8.0 and 7.0 -18.5 GHz bands are probed during distinct experiments, each using a pulse that sweeps across the appropriate frequency band.…”

“…[39][40][41] A tendency for the described experimental conditions to generate products of the fragmentation of precursors has been noted during recent studies of imidazole and pyrazole derivatives. 18,42,43 Godfrey et al…”

On assessing functional errors in density functional theory using atomisation energies and electric field gradients

“…While density differences calculated from an accurate ab initio approach and a more approximate density functional are conceptually appealing, the values obtained are very sensitive to the exact details of the calculations, for example sampling points [3]. An alternative way of assessing the accuracy of the density is through electric field gradient (EFG), for which highly accurate values can be derived from microwave spectroscopy [19]. Consequently, in the present work, we use EFGs as a probe of the electron density and investigate the correlation between energy and density errors for a range of energetically-motivated conventional density functional approximations for five molecules and complexes; CuCl, H 2 OÁÁÁCuCl, ArÁÁÁCuCl, OCÁÁÁCuCl, and H 2 SÁÁÁCuCl.…”

Rotational Spectroscopy Probes Lone Pair···π-Hole Interactions in Hexafluorobenzene-Tertiary Alkylamines Complexes

“…57,58 This analysis relies on the extended Townes-Dailey (ETD) model directly relating to the quadrupole coupling tensor. 59 Within the ETD model, only the valence electron population in the p-orbitals contributes to the nuclear quadrupole coupling tensor. If the nuclear quadrupole coupling principal axes and the axes of the p-orbitals coincide, the populations (P) and the nuclear quadrupole coupling constants (χ) are related by equations which contain the population of the orbital and the oneelectron contribution to the coupling tensor, χ 0 .…”

“…Based on the analysis of the nuclear quadrupole coupling constants, the spectroscopic evidence of the formation of a lp-π interaction in these complexes can be provided using the valence p-orbital population anisotropy. , This analysis relies on the extended Townes-Dailey (ETD) model directly relating to the quadrupole coupling tensor . Within the ETD model, only the valence electron population in the p-orbitals contributes to the nuclear quadrupole coupling tensor.…”