Geometries and properties of guanine–BH3 complex: an investigation with density functional theory (DFT) method

Journal of Molecular Structure (Theochem) 682 (2004) 47–53 www.elsevier.com/locate/theochem

Geometries and properties of guanine –BH3 complex: an investigation with density functional theory (DFT) method Shiguo Zhanga,b, Hong Lib, Pin Yanga,*, Sidian Lic a

Chemical Biology and Molecular Engineering Laboratory of Education Ministry, Institute of Molecular Science, Shanxi University, Taiyuan 030006, China b Department of Computer Science, Binzhou University, Binzhou 256600, China c Department of Chemistry, Institute of Material Science, Xinzhou Teachers’ University, Xinzhou 034000, China Received 23 February 2004; accepted 5 April 2004 Available online 17 July 2004

Abstract Geometries and binding energies are predicted at B3LYP/6-311 þ G* level for the guanine –BH3 systems and seven conformers with no imaginary frequencies have been obtained. Single energy calculations using much larger basis sets (6-311 þ G(2df,p)) and the analyses for the combining interaction between BH3 and guanine with the theory of atom-in-molecules (AIM) have been performed. The results indicate that four conformers (a) – (d), in which the pyridine-type nitrogen atoms or carbonyl oxygen or nitrogen of amino group offers their lone pair electron to the empty p orbital of born atom, are the stable ones with the stabilization energies of 29.55, 26.10, 21.76 and 18.39 kcal/mol (with BSSE correction), respectively, while the remaining conformers in which the pyrrole-type nitrogen atom or carbon atom offers p electron to the empty p orbital of born atom are fairly unstable. Four of the seven conformers, (a), (c), (d) and (f), exist dihydrogen bonds between guanine and BH3 fragments and conformer (a), in which pyridine-type nitrogen interacted with boron and two dihydrogen bonds were formed in it according to the analysis from AIM, is the most stable one. q 2004 Elsevier B.V. All rights reserved. Keywords: Density functional theory; Intermolecular interaction; Guanine–BH3 complexes

1. Introduction Intermolecular interaction plays an important part in intermolecular recognition processes essential to most biological systems. It has been found that a lot of physical and chemical phenomena are closely related to the intermolecular weak interaction or the formation of various complexes, which are combined with charge transference, hydrogen bond, various van der Waals forces, etc. [1 – 5]. Among them, charge transference interaction is one of the most important styles of interaction and the most important methods of energy transference in the biology systems. In these systems there exist various kinds of weak interactions and such systems are often called supermolecular systems, for example, enzyme –substrate complexes and medicine – acceptor complexes. At the same time, this interaction is also important in determining the structures and activities of organic, organometallic and biological molecules. * Corresponding author. Tel./fax: þ 86-351-7011022. E-mail address: [email protected] (P. Yang). 0166-1280/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2004.04.064

The studies about the intermolecular interaction of biological molecules are basis to learn special functions and the mechanism of recognition processes. Bases of nucleotides are the most important portions to build up the DNA or RNA molecules. All the bases are heterocyclic compounds that contain conjugation systems including nitrogen, oxygen and carbon atoms. Weak interactions exist around these bases widely and they are important to life phenomenon. Boron contained compounds are electron deficient compounds and have been extensively used as catalysts in chemical reactions. There have been many experiments and theoretical reports [6 – 8] about the donoraccepter complexes involving BH3, BF3 and BCl3 etc, which are called Lewis acid – base complex. Recently, it has been found that the complexes of nucleotides or peptides with boron hydride have anti-tumor activities and the investigation about it has been made extensively [9,10]. As mentioned above, it is meaningful to study the nature of the interaction in these systems. The complexes of bases with various small molecules, such as H2O,CH3OH, CO or metal cations have been reported in the literatures [11]. But, to

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the best of our knowledge, the complexes of bases with the boron hydride have not been studied experimentally or theoretically except for our previous papers on the theoretical studies of uracil– BH3 and adenine – BH3 systems [12,13]. Molecular conformation plays a crucial role in selectivity and function of biologically active molecules, such as neurotransmitters and enzymes. Thus, all the possible conformations of the system being studied are discussed. It is well known that electron correlation is important in determining the geometries and other properties of intermolecular interaction complexes. For this purpose, the MP2 method, the most popular method including electron correlation, has become a standard method to study intermolecular interaction. But due to computer time and size limitations MP2 often applies to relatively small systems [14 –16]. In contrast, the DFT is a successful alternative for post Hartree – Fock quantum chemical methods in many field of research. It not only involves electron correlation but also saves time as compared to MP2, thus it affords a possible way to study large systems and has been successfully applied to predict the geometries and properties of intermolecular interaction [17,18]. In this paper we have chosen B3LYP method of DFT to study the guanine – BH3 system.

2. Computational methods All calculations have been performed using GAUSSIAN 03 program [19] with the Becke three-parameter hybrid exchange functionals and Lee – Young – Parr correlation functionals (B3LYP) of DFT. All the possible orientations of BH3 towards guanine have been fully optimized at B3LYP/6-31G* level and seven conformers corresponding to the minimum energy points, at which the harmonic frequency analysis have been carried out and the complexes have no imaginary frequency, at the molecular energy hypersurface have been obtained. The obtained conformers have been reoptimized using B3LYP/6-311 þ G* method and vibrational frequencies have been recalculated at the same level so as to confirm the results and improve the reliability of the results as the diffuse function was added in the later calculations. Single energy calculations using much larger basis sets (6-311 þ G(2df,p)) have also been carried out on the seven optimized conformers. The analyses for the intermolecular interactions between BH3 and guanine with the atom-in-molecule theory (AIM) have been performed [20,21]. Binding energy ðDe Þ is defined as the difference between the energy of the complex and the energies of its fragments. For this system it can be expressed as follows De ¼ EðC5 N5 H5 O – BH3 Þ 2 EðC5 N5 H5 OÞ 2 EðBH3 Þ The De corrected for the basis set superposition error (BSSE) was evaluated [22]. Finally, the correction ZEP

and vibration frequencies were applied with a scaling factor of 0.975 referred from literature [23].

3. Results and discussion Seven conformers, (a) –(g), were obtained and their optimized geometry and atomic labels are shown in Fig. 1. Four of them (a) – (d) have s –p type interactions while the remanding three (e) – (g) exhibit p –p type interactions and four of them, (a), (c), (d) and (e), have formed dihydrogen bonds between guanine and BH3 fragments. The optimized geometries parameters of the seven complexes were listed in Table 1 and their binding energies were listed in Table 2. Frequency shifts of the monomers in complexes are shown in Table 3. Intermolecular bond critical points (bcps) from AIM analysis are also shown Fig. 1 and their topological properties of electron density distributions at the bond critical points between boron and guanine and the dihydrogen bonds calculated with AIM theory are in Table 4. The four conformers in which the pyridine-type nitrogen or carbonyl oxygen or nitrogen atom of amino group offers its lone pair electron to the empty p orbital of born atom are the relatively stable ones. The conformers in which the pyrroletype nitrogen atom or carbon atom offers p electron to the empty orbital of born atom are fairly unstable. The conformer (a), in which pyridine-type nitrogen interacted with boron and two dihydrogen bonds were formed in it, is the most stable one. As mentioned above, the B3LYP methods of DFT with moderate basis sets and diffuse function can give a reliable outcome, the discussion about guanine – BH3 system in this paper was be done using B3LYP/6-311 þ G* method. 3.1. Geometries From Fig. 1, it can be seen that the pyridine-type nitrogen atom connected to boron in conformer (a), (b) and the two conformers are the most stable ones of all the conformers obtained in the present research. In conformer (a), the N3 – B ˚ and the structure of BH3 changes from distances is 1.618 A plane to pyramid (/HBH < 111.78). The C2 –N3, N3 – C4 distances of guanine lengthened from 1.308, 1.356 to 1.332, ˚ , and the C1 – C2, C4 –C5 and C4 – C9 distances are 1.371 A diminished slightly but C2 – N3 distance shortened evidently ˚ . The B – H bond length changes from 1.376 to 1.349 A ˚ in boron hydride to 1.210 and 1.221 A ˚ . The from 1.192 A ˚, a H13 –H18 and H15 – H20 distances are 2.095 and 2.031 A value less than twice the van der Waals radius of hydrogen ˚ [24], and therefore two dihydrogen bonds equal to 2.38 A between N9 –H13 and B – H18, N11 – H15 and B –H20 were formed. As the formations of dihydrogen bonds, the N9 – H13, N11 –H15 distances lengthened from 1.009 to 1.014 and ˚. 1.013 A The boron atom connected to N7 atom in conformer (b) and located in the same plane with guanine. The N7 – B

S. Zhang et al. / Journal of Molecular Structure (Theochem) 682 (2004) 47–53

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Fig. 1. Optimized structures (B3LYP/6-311 þ G*) and atomic labels of conformers. Intermolecular bond critical points (bcps) from AIM analysis are also shown.

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Table 1 Principal geometry parameters for free C5N5H5O and BH3 and for C5N5H5O– BH3 complex, at B3LYP/6-311 þ G* level Geometry parameters

C5N5H5O/BH3

(a)

(b)

(c)

(d)

(e)

(f)

(g)

R(N1 –C2) R(N1 –C6) R(C2 –N3) R(C2 –N11) R(N3 –C4) R(C4 –C5) R(C4 –N9) R(C5 –C6) R(C5 –N7) R(C6 –O10) R(N7 –C8) R(C8 –N9) R(B–Y)a R(B–H18) R(B–H19) R(B–H20) A(H –B–H)

1.370 1.439 1.308 1.376 1.356 1.393 1.369 1.438 1.381 1.213 1.304 1.384 – 1.192 1.192 1.192 120.0

1.360 1.447 1.332 1.349 1.371 1.389 1.359 1.437 1.379 1.209 1.304 1.384 1.618 1.210 1.218 1.221 111.7

1.368 1.442 1.312 1.368 1.350 1.389 1.373 1.442 1.389 1.207 1.313 1.367 1.618 1.215 1.209 1.209 113.0

1.373 1.390 1.312 1.365 1.348 1.399 1.367 1.416 1.380 1.254 1.303 1.387 1.605 1.198 1.216 1.224 112.9

1.351 1.443 1.295 1.450 1.363 1.391 1.368 1.446 1.376 1.208 1.307 1.380 1.676 1.213 1.207 1.205 113.9

1.438 1.523 1.292 1.360 1.362 1.390 1.364 1.430 1.381 1.198 1.303 1.386 1.802 1.196 1.210 1.201 115.6

1.366 1.440 1.317 1.365 1.339 1.376 1.428 1.440 1.399 1.211 1.280 1.447 1.808 1.205 1.200 1.200 116.1

1.368 1.438 1.312 1.371 1.350 1.404 1.364 1.446 1.389 1.210 1.299 1.388 2.537 1.193 1.189 1.195 119.6

Distances are in Angstrom and angles are in degree. Y, which is an atom in the guanine fragment, is the closest atom of guanine fragment, to boron, A(H –B–H) is the average of the three H–B–H angles in BH3 fragment. a

˚ , which is the same as that of N3 – B in distance is 1.618 A conformer (a). The most evident changes of bond length in guanine bone were C5 –N7, N7 – C8 and their distances ˚ , respectively. The distance lengthened by 0.008 and 0.009 A ˚ . The of C8 – N9 of guanine diminished from 1.384 to 1.357 A structure of BH3 has changed to pyramid from plane (/HBH < 113.08) and their B – H bond lengthened from ˚. 1.192 to 1.206 and 1.216 A In conformer (c), boron atom connected to O10 and the ˚ , which is shorter than that of B – O distance is 1.605 A B – N in conformer (a) and (b). The changes of B –H bonds are the same as that in conformers (a) or (b), that is B – H distances lengthened and the BH3 have changed its structure to pyramid (/HBH < 112.98). As carbonyl

oxygen O10 offers its lone pair electron to connect to boron atom, the C6 – O10 distance lengthened from 1.213 ˚ . At the same time, the C5 – C6 and N1 – C6 to 1.254 A ˚. distances diminished from 1.438, 1.439 to 1.416, 1.390 A ˚ , and a dihydrogen The H14 –H20 distance was 1.977 A bond between N1 – H14 and B– H19 was formed as shown in Fig. 1(c). As the formations of dihydrogen bonds, the N1 –H14 and B – H19 distances lengthened from 1.009, ˚ . A syn – anti isomer conformer of 1.192 to 1.020, 1.224 A (c) about the bond C6yO10 has been found. It was also a ˚ , that local energy minimum. The B – O distance is 1.664 A is a little longer than that in (c) and this conformer possessed a total energy 2 569.3377426 a.u., which is higher than (c) by 9.06 kcal/mol.

Table 2 Binding energies of conformers (the value in the parentheses is BSSE-corrected De and that under the binding energy is De with zero-point energy correction) Conformer

(a) (b) (c) (d) (e) (f) (g) a

B3LYP/6-311 þ G(2df,p)a

B3LYP/6-311 þ G*

B3LYP/6-31G* Etotal (a.u.)

2De (kcal/mol)

Etotal (a.u.)

2De (kcal/mol)

Etotal (a.u.)

2De (kcal/mol)

2569.0334765 2568.8823900 2569.0275108 2568.8766540 2569.0163841 2568.8661060 2569.0070006 2568.8550120 2568.9831427 2568.8334350 2568.9809742 2568.8320100 2568.9788512 2568.8316540

36.18 (33.09) 31.87 32.43 (29.33) 28.27 25.46 (21.89) 21.66 19.57 (16.60) 14.7 4.61 (2.15) 1.17 3.25 (1.27) 0.276 1.92 (0.514) 0.053

2569.3648913 2569.2160340 2569.3592433 2569.2105270 2569.3521949 2569.2036300 2569.3487193 2569.1980820 2569.3240119 2569.1754770 2569.3215743 2569.1737600 2569.3189411 2569.1732710

30.33 (29.55) 27.7 26.79 (26.10) 24.24 22.37 (21.76) 19.92 20.19 (18.39) 16.44 4.69 (3.35) 2.26 3.162 (2.03) 1.18 1.51 (0.97) 0.88

2569.4164000

31.45

2569.4101659

27.54

2569.4043749

23.91

2569.3966776

19.08

B3LYP/6-311 þ G(2df,p)//6-311 þ G*.

2569.3726654

4.019

2569.3705176

2.672

2569.3681483

1.186

S. Zhang et al. / Journal of Molecular Structure (Theochem) 682 (2004) 47–53

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Table 3 Selected frequency shifts of BH3 and guanine in conformer (a)–(g) of the guanine–BH3 complex (B3LYP/6-311 þ G*) BH3/C5N5H5O

(a)

(b)

(c)

(d)

(e)

(f)

(g)

2594 (136.6) 2594 (136.6) 2467 (0)

2199 (209.5) 2221 (177.2) 2138 (142.1)

2187 (202.8) 2205 (148.6) 2131 (186.2)

2115 (229.6) 2258 (196.1) 2190 (172.5)

2146 (191.8) 2182 (217.1) 2116 (79.7)

275 (113.5) 2137 (116.4) 299 (123.9)

2100 (140.9) 2126 (153.2) 272 (84.3)

0 (84.1) 235 (115.9) 215 (14.1)

C5N5H5O n4 3572 (29.9) n5 3470 (40.6) n6 1642 (287.5) n7 3539 (60.5) n8 3483 (38.3) n9 3148 (1.2) n10 1738 (758.9)

10 (78.6) 232 (119.8) 21 (427.9) 221 (105.5) 6 (45.7) 8 (0.3) 17 (814)

15 (37.8) 10 (41.6) 21 (346.9) 8 (96.6) 1 (68.8) 49 (9.1) 18 (590.7

22 (44.8) 16 (83.7) 24 (482.2) 22 (77.5) 2124 (219.9) 6 (0.3) 269 (822.6)

2150 (52.6) 2115 (27.3) 21 (97.4) 24 (73.1) 264 (157.3) 6 (0.5) 16 (720.0)

221 (52.5) 225 (72.8) 16 (596.8) 25 (75.1) 275 (40.3) 7 (0.6) 57 (601.6)

19.4 (40.8) 7 (30.8) 23 (351.1) 2116 (56.7) 1 (87.0) 13 (2.5) 10 (750.6)

8 (34.6) 6 (47.0) 0 (283.3) 22 (67.6) 0 (50.7) 2 (1.1) 9 (659)

BH3 n1 n2 n3

All frequencies are in cm21 and IR intensities (km/mol) are enclosed in parentheses.

In conformer (d) the boron hydride lies over the amino group of guanine and the distance between B and N11 was ˚ , It is longer than that of B – N in conformer (a), (b). 1.676 A The distance of C2 –N11 was lengthened markedly from ˚ and the distances of N1 – C2, C2 – N3 1.376 to 1.450 A ˚ . The BH3 diminished from 1.370, 1.308 to 1.351, 1.295 A changed its structure from plane to pyramid and its B – H bond lengthened also. There was a dihydrogen bond in conformer (d) between bond N1 –H14 and B – H18 (Fig. 1(d)), ˚. and the H14 –H18 distance is 2.049 A The boron hydride was located over the pyrrole-type nitrogen atom N1 of guanine in conformer (e) and the ˚ . The most evident changes of distance of B – N1 is 1.802 A bond length in guanine bone were that N1 – C2, N1 –C6 ˚.A distances lengthened from 1.370, 1.439 to 1.438, 1.523 A dihydrogen bond has been formed between N1 –H16 and B – H18 so the N1 – H14 and B –H18 bond were lengthened. The N1 atom has changed its plane structure to pyramid and made H14 moved to the opposite direction of BH3. Still, just as conformer (e), the boron hydride lies over pyrrole-type nitrogen atom N9 of guanine in conformer (f) and the ˚ . The distances of C4 – N9 and distance of B –N9 is 1.808 A ˚, C8 – N9 have been lengthened by 0.059 and 0.063 A

respectively and the N9 has changed its plane structure to pyramid as N1 did in (e). There existed no dihydrogen in conformer (f) as in (e). The distance of B – C5 in conformer ˚ . The bond lengths of guanine and BH3 in this (g) is 2.537 A conformer are almost unchanged because the interaction of boron and C5 is weak. 3.2. Binding energies and stabilities Anane group have calculated the complex systems H3B–NH3, H3B – NH2Me, H3B – NHMe2 and H3B –NMe3 at MP2(full)/6-31G* level [25] and have given results that the binding energies of these complex are: 26.0 (21.7), 31.9 (34.9), 35.2 (36.4) and 36.3 (38.3) kcal/mol. The data in brackets is their experimental binding energies come from reference [26]. The most stable complex system of pyrimine – BH3 has been calculated in Ref. [5] with MP2/6-31 þ G* and B3LYP/6-31G* and the calculated binding energies is 33.9 and 31.4 kcal/mol, respectively. Compared the results mentioned above with the data listed in Table 2, it can be seen that the calculated results in this paper are reliable. The binding energies in Table 2 indicate that the conformers (a) – (d) were relatively stable conformers as they were all

Table 4 ˚ about the interactions between guanine and BH3 in their complexes Bond critical point data in atomic unit and bond length in A Conformer

bcp

Interaction

rða – bÞ

r

Laplacian r

1

Hb

(a)

bcp1 bcp2 bcp3 bcp1 bcp1 bcp2 bcp1 bcp2 bcp1 bcp2 bcp1 bcp1

B –N3 H13 –H19 H15 –H20 B –N7 B –O10 H14 –H19 B –N11 H14 –H18 B –N1 H16 –H18 B –N9 B-C5

1.618 2.095 2.031 1.618 1.607 1.977 1.676 2.049 1.806 2.179 1.813 2.540

0.11104 0.01219 0.01618 0.10718 0.09405 0.01963 0.09610 0.01369 0.07175 0.01099 0.06905 0.01999

0.11275 0.01052 0.01365 0.12235 0.12998 0.01586 0.10044 0.01132 0.05968 0.00963 0.05942 0.00987

0.0355 0.7636 0.4264 0.0138 0.0490 0.4902 0.0584 0.5920 0.6565 1.5615 4.4720 0.3877

20.18073 20.00861 20.00994 20.18386 20.17364 20.01422 20.15445 20.18920 20.09927 20.00800 20.09660 20.01096

(b) (c) (d) (e) (f) (g)

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formed with the action that nitrogen or oxygen atom offered its lone pair electron to the empty p orbital of boron atom. Conformer (a) is the most stable conformer and as it has no imaginary frequency at the same time thus it is the global minimum. Besides there is an s– p interaction with charge transference from nitrogen to boron in conformer (a), two dihydrogen bonds were formed whereas there was no such bonding in conformer (b). Although the s –p interactions existed both in (a) and (b) and they have the same N –B distances (Fig. 1) as the former has two dihydrogen bonding interactions it is more stable than (b) by 3.45 kcal/mol (B3LYP/6-311 þ G*, BSSE corrected) and is the most stable conformer of guanine – BH3 system. All the binding energies listed in Table 2 was obtained using the same method, but different basis sets, which is 6-31G* or 6-311 þ G* or 6-311 þ G(2df,p) and all the calculations have received the relatively consistent results. For conformers (a) – (d), the B –N or B –O distance is relatively short and their BH3 have change its shape greatly, the interaction between boron and nitrogen or oxygen is strong. Then they are relatively stable systems. From Tables 1 and 2, for conformer (e) –(g), the B – N or B – C distance is large and the shapes of their BH3 almost have no change. So, their binding energies are relatively small and they are not stable systems. Although both N1 and N9 have the similar properties and chemical environment in conformer (e) and (f), the former one was more stable than later by 1.32 kcal/mol (B3LYP/ 6-311 þ G*, BSSE corrected) because there is a dihydrogen bond in the former. 3.3. Vibration frequency Among the three vibrational frequencies of BH3, n1 and n2 can be approximately described as the antisymmetric stretching vibration of BH3, n3 as the symmetric stretching mode. From Table 3 it can be seen that the n1 ; n2 and n3 decreased (red shifts) in all the conformers in this research. The antisymmetric BH3 stretching splits into two frequencies in complexes. Although the n3 mode is infrared inactive in the monomer, it becomes activated in the complex, but the IR intensities of it are much lower than that of antisymmetric mode in most conformers. In conformer (a), which is the most stable one, the frequencies of BH3 suffered the largest red shifts while the red shifts in conformer (g), which is the most unstable conformer, are the smallest. It is obvious that according the sequence of (a) – (g), their binding energies of the seven conformers decrease, and their B –H bond lengthened and then the red shifts of B –H bond stretching vibration diminished. The selected vibration frequency shifts of C5N5H5O have been also showed in Table 3. n4 and n5 can be approximately described as the antisymmetric and symmetric stretching of NH2 group, respectively, n6 as the scissor mode. n7 and n8 can be approximately described as the stretching mode of N9 – H13 and N1 – H14, respectively.

n9 ; n10 are the approximate description of stretching modes for C6yO10 and C8 – H12. From Table 3, we can see that the as two dihydrogen bond were formed in conformer (a), its n5 ; n7 underwent red shifts but n4 suffered a blue shift. In conformer (b), all the frequencies listed in Table 3 were blue-shifting but n6 has a little red shift. As O10 connected to boron and N1 –H14 took part in a dihydrogen in conformer (c), n8 and n10 suffered very large red shifts. In conformer (d), as N11 connected to boron, n4 and n5 suffered very large red shifts. Still, n8 underwent a red shift as a dihydrogen bond was formed with N1 –H14. In conformer (e), as N1 offered its lone pair electron to boron the stretching mode of N1 – H14 shifted to red side but n10 ; the approximate description of stretching modes for C6yO10 underwent a blue shift. Just like conformer (e), n7 suffered a large red shift in (f). As the interaction in conformer (g) is very weak, all the frequencies only underwent small shits. The interaction between boron and nitrogen or oxygen or carbon atom of guanine is important in complexes. From Table 3, it can be seen that the frequency trend of the conformers is approximately in accordance with the stability of complexes, viz. the larger the frequency is, the more stable the conformer is. 3.4. AIM analysis A topological analysis of the electron density was performed using Bader’s theory of atoms-in-molecules (AIM) [20,21]. The analysis has been applied to studies of properties of a variety of interaction between atoms especially hydrogen-bonded systems exhibiting both conventional and nonconventional hydrogen bonds [27,28]. Specially, the topology of the electron density of bcps for the binding interaction between guanine and BH3 was analyzed in this paper (Table 4). The electron density, rðrÞ; at the bcp is related to the bond order and therefore the bong strength [29]. The eigenvalues associated with the second derivatives of shape of r at the bcp, l1 ; l2 and l3 ; indicate how rapidly the density changes on moving away from the bcp and represent the curvatures of electron density along different directions. For a normal single bond, such as the C –C bond in ethane, the two negative curvatures (l1 and l2 ), which are perpendicular to the bond line, are approximately equal. However, if there is a double bond one curvature (in the p-bond) will be much smaller than the other. This difference may be described by the ellipticity, 1; of the bond, which is defined as 1 ¼ l1 =l2 2 1; in which l2 is the curvature of smaller magnitude. For a single bond, l1 < l2 ; therefore, 1 < 0: for a double bond, l1 . l2 ; therefore, 1 . 0: The trace of or the sum P of the second partial derivative values of Laplacian r ¼ j¼1;3 ›2 rðrÞ=›x2j at the bcp is negative for covalent interaction and is positive for an interaction between closed-shell systems [30]. Hb is the local energy density of bcp. These measures are used in the following for the description of the interaction between guanine and BH3.

S. Zhang et al. / Journal of Molecular Structure (Theochem) 682 (2004) 47–53

Table 4 presents the bond critical points (bcp) between guanine and BH3 in all the seven complexes. It can be seen that some of the complexes possess more than one bcps between guanine and BH3. Besides the bcp of B– X (X ¼ N, O or C) there exists one or two bcps, which present the dihydrogen bond between guanine and BH3 in these complexes. The data in Table 4 showed that the r of bcps about B –X (X ¼ N, O or C) decreased from conformer (a) –(g) as the consequence of binding energy decreased. The 1 of B – X in (a) –(d) equal to 0.0138 –0.0584, that is very close to zero. But in (e) –(g) the data of 1 are relatively large and mean that the B – X interactions are weak. All the separations of hydrogen atoms between which dihydrogen bond was found are less than twice of the van der Waals radius of hydrogen. The most characteristic features of dihydrogen bond in Table 4 were found that the distance H – X lengthened in dihydrogen bond and their vibrational frequencies suffered red shift as mentioned above. All the 1 in Table 4 have positive value mean that the interaction between guanine and BH3 have the character of close shell.

4. Summary We performed DFT calculations on the guanine – BH3 system. Seven different conformers corresponding to the minimum points at the molecular energy hypersurface were found. The four conformers in which the pyridine-type nitrogen or carbonyl oxygen or nitrogen atom of amino group offers its lone pair electron to the empty p orbital of born atom are the relatively stable ones. The conformers in which the pyrrole-type nitrogen atom or carbon atom offers p electron to the empty orbital of born atom are fairly unstable. According to the AIM analysis, dihydrogen bonds between guanine and BH3 fragments have been formed in some conformers and made contribution to the stability of the complexes. Conformer (a), in which pyridine-type nitrogen interacted with boron and two dihydrogen bonds were formed, is the most stable one. Frequency analysis suggested that the stretching vibration frequency of BH3 underwent a red shift in complexes and all the frequencies of N – H bonds, in which the hydrogen formed dihydrogen bond, suffered a large red shift and their bond lengthened.

Acknowledgements This work was supported by the National Natural Science Foundation of China (20171031) and The foundation for key teachers in university of education office of Shandong province, China.

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