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

Graham A. Cooper, Chris Medcraft, Josh D. Littlefair, Thomas J. Penfold, Nicholas R. Walker*

*Corresponding author for this work

Research output: Contribution to JournalArticleAcademicpeer-review


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, χaa(X) and χbb (X)-χcc (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 CH3X and CF3X.

Original languageEnglish
Article number214303
JournalJournal of Chemical Physics
Issue number21
Publication statusPublished - 7 Dec 2017


The authors thank the European Research Council for the postdoctoral fellowships awarded to G.A.C. and C.M. and for project funding (Grant No. CPFTMW-307000). We are grateful to Professor Anthony C. Legon for a critical reading of the manuscript, to Professor Mike Waring of Newcastle University for alerting us to the importance of halopyrazoles in biochemical structure determination, to Dr. Alistair Henderson of NewChem Technologies for preparation of the deuterated samples, and to Newcastle University for the award of a Research Excellence Academy studentship to J.D.L.

FundersFunder number
European Research CouncilCPFTMW-307000


    Dive into the research topics of 'Halogen bonding properties of 4-iodopyrazole and 4-bromopyrazole explored by rotational spectroscopy and ab initio calculations'. Together they form a unique fingerprint.

    Cite this