Experimental and theoretical structural parameters of the glycine C ˙ H 2 –NH 2 radical V. Chis ¸ a, * , M. Brustolon b , C. Morari c , O. Cozar a , L. David a a Babes-Bolyai’’ University, Department of Physics, 1, Kogalniceanu, Cluj-Napoca RO-3400, Romania b Universita degli Studi di Padova, Dipartimento di Chimica Fisica, Via Loredan 2 I-35131, Padova, Italy c Institute of Isotopic and Molecular Technology, Cluj-Napoca RO-3400, Romania Received 24 August 1998; accepted 11 November 1998 Abstract An experimental and theoretical analysis is reported for the geometrical structure and unpaired spin density of the C ˙ H 2 –NH 2 radical produced in a g -irradiated single crystal of glycine at room temperature. Geometrical parameters obtained by using the two-center dipole approximation indicate that the radical is not planar at the carbon site. Ab initio calculations were performed in order to estimate the structural parameters and unpaired spin density of the radical C ˙ H 2 –NH 2 in an anisotropic matrix. A good agreement was obtained between the calculated and observed values. Using the Saturation Recovery (SR) method in the temperature range 190–290 K we have obtained a set of spin lattice relaxation times, attributed to the reorientation of the amino group of the NH 3 –C ˙ H–COO - radical, simultaneously produced in g -irradiated solid glycine with C ˙ H 2 –COO - and C ˙ H 2 –NH 2 radicals.  1999 Elsevier Science B.V. All rights reserved. Keywords: Ab initio calculations; Structural parameters; Relaxation time 1. Introduction The free radicals produced by high-energy irradia- tion of glycine ( + NH 3 –CH 2 –COO - ) have been the subject of many EPR and ENDOR studies. In a recent study [1], besides the two well known radicals C ˙ H 2 – COO - (radical A) and + NH 3 –C ˙ H–COO - (radical B), formed by deamination and deprotonation, respec- tively, a new radical, C ˙ H 2 –NH 2 , (radical C) has been identified in a room temperature g -irradiated single crystal of glycine. In this work, the three hyperfine coupling tensors of the radical C obtained by ENDOR spectroscopy were carefully examined and the structure, bond lengths and unpaired spin density of this radical are presented. In order to obtain further information on the relaxa- tion behavior of the glycine radicals, we also performed a pulsed EPR study on a g -irradiated single crystal of glycine owing to the fact that rather few results regarding the relaxation times of these radicals are reported till now [2–4]. Electron spin relaxation times were measured by the Saturation Recovery (SR) method. 2. Experimental Single crystals of glycine were grown from aqueous solutions by slow evaporation at room temperature and then irradiated by g rays from a 60 Co source Journal of Molecular Structure 482–483 (1999) 283–286 0022-2860/99/$ - see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0022-2860(98)00854-0 * Corresponding author. Tel.: + 40 64 194 315; fax: + 40 64 191 906. E-mail address: vchis@hera.ubbcluj.ro (V. Chis ¸)