Theoretical study of hyperfine coupling constants of uracil, cytosine and their halogenated derivatives in triplet state Xin-Juan Hou, Minh Tho Nguyen * Department of Chemistry, University of Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium Received 10 June 2004; accepted 27 September 2004 Available online 30 October 2004 Abstract A detailed theoretical study of the molecular geometries, singlet–triplet energy gaps, and hyperfine coupling constants of uracil, cytosine, and their 5-X and 6-X halogeno-substituted derivatives (X = F, Cl, and Br) was performed. The geometries were optimized both in the ground singlet and lowest triplet excited states using the (U)B3LYP/6-311++G(d,p) method. While ground state geom- etries are planar, the corresponding excited-state geometries were predicted to be highly nonplanar. The vertical triplet states of the halouracils and halocytosines are calculated to be centered at 3.3–4.0 eV above the corresponding singlet ground state. Geometry relaxation leads to an energy gain of 0.5–0.7 eV in triplet 5-X uracil and 5-X cytosine, 1.0–1.4 eV in triplet 6-X uracil and 6-X cyt- osine. The hyperfine coupling constants of triplet state of halouracils and halocytosines have been predicted using UB3LYP method and different basis sets. As for a calibration, calculations were also considered for cytosine-1-yl and uracil-1-yl radicals for which experimental hyperfine constants are available. The spin density is mainly located at C 5 and (or) C 6 atoms of the six-membered rings. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction Although the triplet states of the nucleic acid bases are of interest when considering excited states of RNA and DNA, relatively little is known about their proper- ties. The precise information about the triplet nucleic bases could be valuable for understanding the intermo- lecular energy transfer for the photoexcitation of DNA. Earlier kinetic studies [1] suggested that the for- mation of the photodimer between uracil, thymine and orotic acid arises from the reaction of one molecule in a triplet state with a second one in its ground state. According to this simple model [1], the triplet state T 1 formed by an intersystem crossing from the photo-ex- cited S 1 state, underwent a self-quenching giving either a deactivation or a formation of photo-products. Spec- troscopic observation of the triplet state of uracil in solution has subsequently been reported [2]. Kinetic studies of the flash photolysis of uracil and thymine, also provided additional evidence for the formation of a high spin transient state [3]. The triplet state interactions, including intermolecular energy and electron transfer, between nucleic acid bases in solution at room tempera- ture were studied by analyzing the transient absorption spectra, and the relative triplet ordering of nucleic acid is cytidine > uridine > guanosine > adenosine > thymi- dine [4]. Abouaf, Pommier and Dunet (APD) [5] studied the excitation of the lowest electronic states of thymine (T) and 5-bromouracil (5-BrU) using electron energy loss spectroscopy (EELS) under electron impact (0–100 eV) and with angular analysis. The new transitions were ob- served at 3.6 eV for T and 3.35 eV for 5-BrU, with an estimated error of ±0.08 eV. APD [5] assigned the new transitions to the lowest-lying triplet states. Due to their short lifetimes and high reactivity, it is often very 0301-0104/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2004.09.034 * Corresponding author. Tel.: +32 163 27361; fax: +32 16 327992. E-mail address: minh.nguyen@chem.kuleuven.ac.be (M.T. Nguyen). www.elsevier.com/locate/chemphys Chemical Physics 310 (2005) 1–9