Conformational aspects of polymorphs and phases of 2-propyl-1H-benzimidazole.

Fco. Javier Zuñiga,Tomasz Breczewski,María Mar Quesada-Moreno,Xabier M. Aretxabaleta,Juan Ramón Avilés-Moreno,Juan Jesús López-González,Jose Elguero,Aurora J. Cruz-Cabeza,Noelia de la Pinta,Rosa M. Claramunt

doi:10.1107/s2052252518011685

Abstract

This paper reports on the polymorphism of 2-propyl-1H-benzimidazole (2PrBzIm) induced by temperature change. Upon heating, an irreversible reconstructive-type phase transition at T = 384 K from the ordered form I (P212121) to a new polymorph, form II HT (Pcam), was observed. The structural transformation between forms I and II involves significant changes in the crystal packing, as well as a key conformational variation around the propyl chain of the molecule. After the first irreversible phase transition, the II HT form undergoes two further (reversible) phase transitions upon cooling at 361 K (II RT) and 181 K (II LT). All three phases (forms II HT, II RT and II LT) have almost identical crystal packing and, given the reversibility of the conversions as a function of temperature, they are referred to as form II temperature phases. They differ, however, with respect to conformational variations around the propyl chain of 2PrBzIm. Energy calculations of the gas-phase conformational energy landscape of this compound about its flexible bonds allowed us to classify the observed conformational variations of all forms into changes and adjustments of conformers. This reveals that forms I and II are related by conformational change, and that two of the form II phases (HT and RT) are related by conformational adjustment, whilst the other two (RT and LT) are related by conformational change. We introduce the term 'conformational phases' for different crystal phases with almost identical packing but showing changes in conformation.

Highlights

Benzimidazoles, indazoles and benzotriazoles are important organic molecules commonly found in biological systems
After this first cooling cycle, the sample was heated from 120 K up to $410 K; during this heating cycle, a significant thermal event was observed at 384 K (DSC runs are shown in Fig. S3 of the supporting information (SI))
This thermal event corresponds to an irreversible solid– solid phase transition to a new form, which we refer to as IIHT

Summary

Introduction

Benzimidazoles, indazoles and benzotriazoles are important organic molecules commonly found in biological systems Besides their biological roles, these heterocyles have received considerable attention in materials science because they form crystalline materials able to exhibit notable conductivity and ferroelectric properties (Horiuchi et al, 2010, 2012). These heterocyles have received considerable attention in materials science because they form crystalline materials able to exhibit notable conductivity and ferroelectric properties (Horiuchi et al, 2010, 2012) The origin of these properties lies in the polar hydrogen-bonded chains present in their crystal structures and in the ability to transfer protons along those hydrogen bonds (Cosby et al, 2015; Pulst et al, 2016; Nagamani et al, 2010).

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