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http://www.paper.edu.cn Chemical Physics Letters 370 (2003) 147–153 Hyperconjugation versus intramolecular hydrogen bond: origin of the conformational preference of gaseous glycine Weizhou Wang a, Xuemei Pu a, Wenxu Zheng a, Ning-Bew Wong b, Anmin Tian a,* a Faculty of Chemistry, Sichuan University, Chengdu, Sichuan 610064, PR China b Department of Biology and Chemistry, City University of Hong Kong, Kowloon, Hong Kong, PR China Received 11 November 2002; in final form 16 December 2002 Abstract Three experimentally detected low-energy conformers of gaseous glycine are selected as models to investigate the origin of the conformational preference of nonionized glycine, employing the atoms in molecules (AIM) and natural bond orbital (NBO) analysis methods. At the B3LYP/6-311++G(3d,3p) theory level, it is found that the importance of intramolecular hydrogen bond was overemphasized in the previous studies and it is hyperconjugation not intramo- lecular hydrogen bond that determines the order and relative energy of the conformers considered in this Letter. Ó 2003 Elsevier Science B.V. All rights reserved. 1. Introduction understanding of the ground state potential energy surface of glycine has proven to be a serious The simplest amino acid, glycine, H2NCH2 challenge to both experiment and electronic COOH, has three internal rotational degrees of structure theory [1–15]. Such an understanding is freedom in its nonionized state: the rotation of the very important because glycine can be regarded as hydroxyl group around the CAO bond, the rota- a prototype for other amino acids, peptides and tion around the CAC bond, and the rotation of proteins. the amino group around the CAN bond. This Previous theoretical calculations mainly focused leads to more than a dozen rotational conformers on the geometries and relative stability of all pos- [1–8] (three low-energy structures Ip, IIn and IIIn sible conformers of glycine [1–15]. The following are depicted in Fig. 1, and the nomenclature is intramolecular hydrogen bonds have been con- taken from [4]; p denotes the planar heavy-atom ventionally assumed to explain the relative stabil- arrangement while n stands for nonplanar heavy- ity of glycine conformers: intramolecular hydrogen atom arrangement). A complete and quantitative bond involving the amino hydrogen and the car- bonyl oxygen, intramolecular hydrogen bond in- volving the carboxylic acid hydrogen and the * Corresponding author. Fax: +86-028-85412907. amino nitrogen and intramolecular hydrogen E-mail address: [email protected] (A. Tian). bond that involves the carboxylic acid hydrogen 0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00072-1 转载 中国科技论文在线 http://www.paper.edu.cn 148 W. Wang et al. / Chemical Physics Letters 370 (2003) 147–153 Fig. 1. Chemical structures and atomic numbering, the corresponding molecular graphs (small dots represent critical points) and some selected molecular orbitals for three glycine conformers Ip, IIn and IIIn, respectively. and the carbonyl group. For instance, the exis- It has been demonstrated recently that hypercon- tence of two equal NAHÁÁÁO H-bonds and one jugation, rather than steric repulsion, is the OAHÁÁÁO bond was suggested in Ip, which was primary cause that determines molecular confor- always regarded as the origin of Ip being the mation in simple organic molecules like ethane lowest-energy form of neutral glycine in the gas [16,17]. Broadly speaking, hydrogen bond belongs [1–15]. It was also stated that IIn has a single hy- to a kind of intermolecular hyperconjugative in- drogen bond between the hydroxyl hydrogen and teraction. In this Letter, we still separate them the amino nitrogen and IIIn has a hydrogen bon- customarily and treat of the subject from the two ded trans-carboxylic acid moiety and a bifurcated aspects, respectively. NAHÁÁÁO bond between the amino hydrogens and The low-energy conformers Ip and IIn of gly- the carbonyl oxygen. These conclusions were em- cine have been identified by microwave spectros- pirically made in terms of interatomic distances. It copy [9–13]. The third conformer (IIIn) has is surprising that almost in all of the studies so far recently been found in the Ar matrix at tempera- published on glycine the information provided by tures below 13 K [8,14]. In this Letter, we selected the electron distribution has been ignored. Elec- the three ÔtrueÕ conformers as models to undertake tron delocalization and its concomitant stabiliza- a detailed investigation on the origin of the con- tion is a natural feature of the molecular orbital formational preference of gaseous glycine. The approach to bonding. A useful chemical concept Ôglycine storyÕ is continued. for electron delocalization is hyperconjugation which is defined as the favourable interaction of a filled or partially filled orbital, typically a r orbi- 2. Computational details tal, with a nearby empty orbital. Hyperconjuga- tion rationalizes certain chemical phenomena in The relative stability and molecular properties terms of filled-orbital–empty-orbital interactions. of the structures under investigation were deter- 中国科技论文在线 http://www.paper.edu.cn W. Wang et al. / Chemical Physics Letters 370 (2003) 147–153 149 mined by the analytic gradient techniques em- However the stability order and the structures of ploying the hybrid density functional method the other glycine conformers depend on the level (DFT) with BeckeÕs three-parameter functional for of theory. For our selected three glycine con- exchange and the Lee, Yang and Parr correlation formers, the optimized structural parameters at the functional (B3LYP) [18,19]. The adequacy of B3LYP(MP2)/aug-cc-pVDZ level of theory have density functional methods for the study of the been reported previously to be in excellent agree- conformational behavior of glycine and other ment with available experimental data [8]. Our amino acids has been the subject of several studies results at the B3LYP/6-311++G(3d,3p) level are [7,8,20,21]. The results obtained have shown that almost the same as those obtained at the the hybrid methods, in particular the B3LYP B3LYP(MP2)/aug-cc-pVDZ level of theory. The method [7,8,20], provide very similar structural energy difference is 0.48 kcal molÀ1 between Ip and parameters and relative stabilities compared IIn and 1.62 kcal molÀ1 between Ip and IIIn. The with MP2 formalism. The standard PopleÕs torsion angle, NCC@O, is )179.35° for IIn and 6-311++G(3d,3p) basis set was used in conjunc- )178.27° for IIIn. It is notable that the deviation tion with the DFT method [22]. The three con- from the planarity of heavy-atom arrangement is formers are all confirmed as true minima on the very small for conformers IIn and IIIn. potential energy surface of gaseous glycine by the presence of only real frequencies after the 3.2. AIM topological analysis of the electronic corresponding vibrational analysis. density The bonding characteristics of the different conformers were analyzed by using the Ôatoms in Recently, the rigorous AIM theory has been moleculesÕ (AIM) theory of Bader [23], which is successfully applied in characterizing hydrogen based on a topological analysis of the electron bonds of different strengths in a wide variety of charge density and its Laplacian. The AIM theory molecular complexes [27,28]. Popelier proposed a has proved itself a valuable tool to conceptually set of criteria for the existence of H-bonding within define what is an atom, and above all what is a the AIM formalism [27,28]. The most prominent bond in a quantum calculation of a molecular evidence of hydrogen bonding is the existence of a structure. The analysis went further with those bond path, containing a bond critical point (BCP) obtained by means of the natural bond obital between the donor hydrogen nucleus and the ac- (NBO) theory of Weihnhold and co-workers [24]. ceptor. The molecular graphs (the ensemble of The NBO analysis will allow us quantitatively bond paths connecting atoms) of the three con- evaluating the above mentioned hyperconjugative formers can be visualized in Fig. 1. Only one reg- interactions and intramolecular hydrogen bonds ular hydrogen bond between N5 and H6AO2 in which would be responsible for the origin of the conformer IIn is found. Table 1 shows the most conformational preference. significant topological local properties at the bond All the calculations were performed using critical points ð3; À1Þ for conformers Ip, IIn and GAUSSIAN 98 program package [25]. The molec- IIIn. The bond lengths are also included. It is ular graphs were generated with the AIM2000 clearly seen from Table 1 that the values of electron (Version 1.0) program [26]. density and Laplacian at N5 ÁÁÁH6 BCP in con- former IIn are also the hydrogen bond type (q 10À2 a.u. and positive r2q). Table 1 also 3. Results and discussion highlights that the OAH bond length in conformer IIn is longer than that in conformer Ip or IIIn, 3.1. Structure and energetics which comes about as a result of the formation of intramolecular hydrogen bond. The formation of All calculations have been consistent in pre- intramolecular hydrogen bond results in the exis- dicting that conformer Ip is the stablest form tence of ring critical point. No other intramolecular among all the conformers of gaseous glycine. hydrogen bond in the three conformers is found. 中国科技论文在线 http://www.paper.edu.cn 150 Table 1 Bond length (in AA)and topological properties (in a.u.) of the electronic charge density at the corresponding bond critical points of glycine conformers Ip, IIn and IIIna Bond Ip IIn IIIn d q r2qe ndq r2qe ndq r2qe n C1AO2 1.203 0.4339 )0.6629 0.1012 1.402 1.338 0.3137 )0.7363 0.0200 0.981 1.354 0.3009 )0.7015 0.0018 0.946 C1AO3 1.354 0.3015 )0.7113 0.0002 0.941 1.201 0.4371 )0.6805 0.1167 1.406 1.204 0.4341 )0.6821 0.1038 1.405 C1AC4 1.523 0.2521 )0.5744 0.1033 0.964 1.536 0.2481 )0.5498 0.1033 0.954 1.526 0.2502 )0.5663 0.1075 0.967 C4AH7 1.092 0.2821 )0.9627 0.0351 0.933 1.090 0.2839 )0.9730 0.0378 0.945 1.091 0.2832 )0.9719 0.0386 0.932 C4AH8 1.092 0.2821 )0.9627 0.0351 0.933 1.090 0.2838 )0.9727 0.0375 0.944 1.092 0.2827 )0.9681 0.0384 0.931 W.

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