# What a Difference a Water Molecule Makes—A Combined Experimental/Theoretical Study on 2,3,5-triphenyl-2H-tetrazol-3-ium Chloride Hydrate in Solution and the Solid-State

## TL;DR

This study combines experiments and theory to explore the structure and properties of a hydrate salt, revealing how water molecules affect its behavior in different states.

## Contribution

The study identifies distinct 'wet' and 'dry' pockets for water molecules and validates theoretical models for accurate predictions.

## Key findings

- At lower temperatures, water molecules occupy different positions around chloride ions, forming 'wet' and 'dry' pockets.
- CAM-B3LYP functional accurately reproduces UV-Vis absorption bands in solution.
- Solid-state calculations using PBE with TS corrections closely match the experimental band gap.

## Abstract

2,3,5-triphenyl-2H-tetrazol-3-ium (TPT) chloride was studied through a combination of theoretical methods and experimental data, revealing structural and physical-chemical properties of the hydrate salt, [TPT]Cl·H2O. The previously reported crystal structure was confirmed, but our study at lower T (100 K vs. 220 K) showed different positions for the two H2O molecules in the unit cell around the chlorides. One of them (Cl1) is found surrounded by the tetrazole units, which we call the “dry pocket”, in contrast to the other, Cl2, which is involved in a hydrogen bonding cluster that consists of chloride and two water molecules, referred to as the “wet pocket”. Hirshfeld surface analyses showed predominant H⋯H interactions, followed by C⋯H interactions (including C–H⋯Cl/O interactions), and H⋯Cl contacts, which represent the C–H⋯Cl2 hydrogen bonds. Density functional theory (DFT) and (time-dependent) TD-DFT calculations on a molecular model of the compound, benchmarking the three functionals B3LYP, CAM-B3LYP, and PBE1PBE, found excellent agreement with experimental solution data when using the CAM-B3LYP function. UV-Vis absorptions observed at 320 nm, 245 nm, and 204 nm (in MeOH solution) were quite accurately reproduced and assigned. The observed bands were assigned to mixed HOMO–n⟶LUMO+m transitions, involving in all cases the LUMO+1 for the most intense band at 245 nm. Solid-state calculations on the GGA (PBE) level of theory using the CASTEP code and including the Tkatchenko–Scheffler (TS) scheme for the description of long-range interactions gave a good match for the calculated electronic band gap in the solid-state of 3.54 eV compared with the experimental value of 3.12 eV obtained through the Tauc plot method.