theoretical study on two types of weak interactions between methylenecyclopropane and xy (x, y = h,...

Theoretical Study on Two Typesof Weak Interactions BetweenMethylenecyclopropane and XY(X, Y ¼ H, F, Cl, and Br)

XIAOYAN LI, LINGPENG MENG, YANLI ZENG, XUEYING ZHANG,SHIJUN ZHENG

Institute of Computational Quantum Chemistry, Department of Chemistry, College of Chemistryand Material Science, Hebei Normal University, Shijiazhuang 050016, China

Received 11 January 2010; accepted 11 February 2010Published online 6 October 2010 in Wiley Online Library (wileyonlinelibrary.com).DOI 10.1002/qua.22663

ABSTRACT: We present theoretical evidence that the two types of interactionsexist in the complexes formed between methylenecyclopropane (MECP) and XY (X,Y ¼ H, F, Cl, and Br). Two seats of XY interacted with MECP are located: (a) is viathe pseudo-p bonding electron pair associated with a CAC bond of the cyclopropanering and (b) is via the typical-p bonding of electron pair of the C¼¼C bond ofMECP. These two types of weak interactions are compared based on the calculatedgeometries, interaction energies, frequency changes, and topological properties ofelectron density. The integration of electron density over the interatomic surface isfound to be a good measure for the strength of weak interaction. Furthermore, thetotal electron density and separated r and p electron densities are also computedand discussed in this article. The separated electron density shows r electron densitydetermined the strength and p electron density influenced the direction of the

Correspondence to: S. Zheng; e-mail: [email protected] grant sponsor: National Natural Science

Foundation of China.Contract grant numbers: 20771033, 20801017, 20973053.Contract grant sponsor: Natural Science Foundation of Hebei

Province.Contract grant numbers: B2008000141, B2008000138.Contract grant sponsor: Education Department Foundation

of Hebei Province.Contract grant numbers: 2009137, 2009138.Contract grant sponsor: Foundation of Hebei Normal

University.Contract grant numbers: L2008B06, L2009Y06.

International Journal of Quantum Chemistry, Vol 111, 3070–3079 (2011)VC 2010 Wiley Periodicals, Inc.

hydrogen/halogen bond. VC 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111:3070–3079, 2011

Key words: theoretical study; weak interaction; methylenecyclopropane; topologicalproperties of electron density; the integration of interatomic surface

Introduction

N oncovalent interactions between moleculesplay a significant role in supramolecular

chemistry, molecular biology, and material sci-ence. The hydrogen bond, the chief mode ofinteraction of which is through electrostatic andcharge-transfer forces, has been the subject ofmany investigations [1–5] and can be said to bethe best characterized type of noncovalent inter-actions. In recent years, the phenomenon of halo-gen bond has been a subject of particular interestwithin the fields of biochemistry [6, 7] and mate-rial science [7, 8]. A halogen bond is a short-range RX-YZ, where X is a halogen atom and Yis often an atom that has a lone pair electron [7–9]. The studies by Brink et al. [9] and subse-quently others [6, 10, 11] showed that the halo-gen atom X in some molecules RX have regionsof positive electrostatic potential on their outersurface, on the extensions of the RAX bond. Hal-ogen bond could thus be explained as the electro-static attraction between the positive potential ofthe halogen and the negative site on another mol-ecule. The region of positive potential on a halo-gen’s surface is often described as a positive r-hole [8], and also been termed the electropositivecrown [6]. The electrostatic attraction between ther-hole and the negative Lewis base is the originof halogen bond. Because of their potential in thedevelopment of new materials and pharmaceuti-cal compounds, much attention has been focusedin recent years to understand the physics andchemistry of the hydrogen/halogen bond bytheoretical [6, 9–17] and experimental [18–20]techniques.

The interactions between methylenecyclopro-pane (MECP) and XY (X, Y ¼ H, F, Cl, and Br)are likely to be of chemical interest for severalreasons [21], such as whether it is possible toform and isolate a complex of the unsaturatedhydrocarbon, the seat of the interaction in thecomplex that might be isolated and the extent ofany electronic charge redistribution attending for-mation of the complex and others. The rotational

spectra of the hydrogen bonded complexesformed between MECP and HF, HCl has beenobtained by pulsed nozzle Fourier-transformmicrowave spectroscopy [22, 23] and that ofMECP with ClF has also been observed by usinga fast-mixing nozzle in combination with a Valle-Flygare microwave spectrometer [21]. Two seatsof the interactions in the complexes are located(shown in Fig. 1): one is the electrophilic endinteract with MECP via the pseudo-p bondingelectron pair associated with a CAC bond of thecyclopropane ring, the other is via the p-bondingelectron pair of the C¼¼C bond.

In this article, we carried out theoretical studieson the intermolecular interactions between MECPand XY (X, Y ¼ H, F, Cl, and Br). The main aimis to investigate and compare the pseudo-p bond-ing with the typical-p-bonding interactions. TheBader’s theory of atom in molecule [24] wasapplied to characterize the bond-critical points(BCPs) and interatomic surface. The geometricaland topological criteria for the existence of thehydrogen bonding, which proposed by Pimenteland McClellan [25] and Popelier and coworker[26, 27] were discussed for the investigated com-plexes. Furthermore, the total electron densityand separated r and p electron densities are alsocomputed and discussed in this article.

Computational Methods

The geometries of the complexes of MECP…XY(X, Y ¼ H, F, Cl, and Br) and the related freemonomers were fully optimized at the B3LYP/6-311þþG (d, p) and MP2 (full)/6-311þþG (d, p)levels. The basis set superposition error (BSSE)[28] corrections were considered in the geometryoptimizations. The equilibrium structures wereexamined by the harmonic vibrational frequencycalculations. All calculations have been performedwith the use of the Gaussian 03 set of codes [29].The topological analyses of electron density werecarried out with AIM2000 [30] and GTA2000 [31]programs, the latter was developed by the

WEAK INTERACTIONS BETWEEN METHYLENECYCLOPROPANE AND XY

VOL. 111, NO. 12 DOI 10.1002/qua INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 3071

authors and registered at QCPE (registered num-ber QCPE-661).

Results and Discussions

GEOMETRICAL PARAMETERS

The halogen bonding is an electrostatic interac-tion between the positive potential on the tip ofthe halogen and the negative potential of thebase. The calculated electrostatic potential surfaceof MECP is shown in Figure 2. There are two sitesof the negative electrostatic potential on the surfa-ces of MECP: One is near the pseudo-p bondingof the cyclopropane ring and the other is near thetypical-p bonding of the C¼¼C bond. Therefore,two seats of XY(X, Y ¼ H, F, Cl, and Br) inter-acted with MECP are located: (a) is the electro-philic end of XY interacts with MECP via the

pseudo-p bonding electron pair associated with aCAC bond of the cyclopropane ring [Fig. 1 (a)];(b) is the electrophilic end interact with MECP viathe typical-p bonding electron pair of the C¼¼Cbond [Fig. 1 (b)].

The geometries of the complexes (b) ofMECP…XY (XY ¼ HF, HCl, ClF) are fully opti-mized and the harmonic frequencies have beencalculated to characterize the stationary point atthe B3LYP/6-311þþG (d, p) and MP2 (full)/6-311þþG (d, p) level. The calculated geometricalparameters and the experimental data availableare gathered in Table I.

From Table I, it can be seen that the bondlengths obtained at B3LYP level are closer to the ex-perimental data [21–23] than those obtained atMP2 (full) level. Furthermore, because the MP2(full) calculation for the complexes are more com-putationally demanding and more time exhaust-ing, the geometries which obtained at B3LYP/6-311þþG (d, p) level are used in the followingdiscussions.

Table II gives the optimized geometry parame-ters of MECP…XY (X, Y ¼ H, F, Cl, and Br). Allof original symmetries of complexes (a) were setto C2v, and they keep unchanged in the optimiza-tions, except for MECP…BrCl turns to Cs. Allcomplexes (b) were set to and kept their Cs sym-metry. Some parameters, such as the bond lengthsof XAY and C1AC2 and the frequency of stretch-ing vibration of XAY have changed after thecomplexes formed. The differences of these

FIGURE 1. Two possible geometries of MECP…XYcomplexes. [Color figure can be viewed in the onlineissue, which is available at wileyonlinelibrary.com.]

FIGURE 2. Electrostatic potential of MECP (yellow isnegative and purple is positive). [Color figure can beviewed in the online issue, which is available atwileyonlinelibrary.com.]

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3072 INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY DOI 10.1002/qua VOL. 111, NO. 12

parameters between the complexes and the mono-mers are also listed in Table II.

In all complexes (a), the X atom of the XY ispointing to the midpoint of C1AC2 bond of thecyclopropane ring. The RC1AC3 and RC3AC4 in thenine complexes are very similar, within theranges of 1.4598–1.4660 and 1.3188–1.3198 A,respectively. The analyses performed here aremainly on the bond length of C1AC2 (RC1AC2)and the distance between X atom and the mid-point of C1AC2 bond (RXA*C1C2). Compared withthe monomer, the bond lengths of the XAY andC1AC2 in complexes (a) increased, such elonga-tions of the XAY and C1AC2 bonds denote that

the weak interaction exists between XY andMECP. Furthermore, there are the following ten-dencies for the changes of geometrical parame-ters: for the same X/Y atom, RXA*C1C2 increases inthe sequence of Y/X ¼ H, F, Cl, and Br.

In complexes (b), the X atom of the XY ispointing to the midpoint of C3¼¼C4 bond ofMECP. The RC1AC2 and RC1AC3 in the nine com-plexes are very similar, within the ranges of1.5409–1.5413 and 1.4568–1.4665A, respectively.Compared to the monomer, the bond lengths ofthe C3¼¼C4 and XAY bond in complexes (b)increased too, just the same as in complexes (a).For the same X atom, the distance between X

TABLE IThe experimental and calculated geometry paramenters of MECP. . .HF, HCl, and ClF (bond length and bondangle in degree).

Complexes RXA*C3C4a RYA*C3C4

b hc /d

X ¼ H, Y ¼ F Experimental [11] 2.138 (8) 3.050 168.2 93.4B3LYP/6-311þþG(d, p) 2.1304 3.0538 168.9 95.6MP2(full)/6-311þþG(d, p) 2.2054 3.1155 167.8 91.8

X ¼ H, Y ¼ Cl Experimental [12] 2.323 (11) 3.569 162.5 90.8 (6)B3LYP/6-311þþG(d, p) 2.3480 3.6374 170.3 96.7MP2(full)/6-311þþG(d, p) 2.420 3.676 165.9 89.7

X ¼ Cl, Y ¼ F Experimental [10] 2.675 (10) 175.1 (4) 92.5 (5)B3LYP/6-311þþG(d, p) 2.5654 176.2 96.4MP2(full)/6-311þþG(d, p) 2.7734 175.3 92.8

aRXA*C3C4 is the distance between X atom and the center of the C3¼¼C4 bond.bRYA*C3C4 is the distance between Y atom and the center of C3¼¼C4 bond.c y is the bond angle formed by Y, X atom and the center of C3¼¼C4 bond.dU is the bond angle formed by Y, the center of C3¼¼C4 bond and C2 atom.

TABLE IIThe calculated geometries parameters and the changes of some selected geometrical parameters, vibrationalfrequencies, and interaction energies of MECP. . .XY (X,Y 5 H, F, Cl, and Br).

Complexes (a) Complexes (b)

RXA*C1C2a RC1AC2

a RXAYa DFreqb DEb RXA*C3C4

a RC3AC4a /a RXAY

a DFreqb DEb

X ¼ H, Y ¼ F 2.1174 1.5641 0.7443 �182.46 �6.03 2.1304 1.3247 95.6 0.7448 �314.28 �13.64X ¼ H, Y ¼ Cl 2.4108 1.5546 0.9300 �75.88 �1.04 2.3480 1.3235 96.7 0.9357 �208.29 �6.56X ¼ H, Y ¼ Br 2.5227 1.5521 1.2925 �60.27 �0.79 2.4068 1.3233 97.1 1.3013 �195.89 �5.24X ¼ F, Y ¼ F 2.9148 1.5490 1.4317 �58.79 �0.26 1.9771 1.3554 98.2 1.4417 �65.93 �27.54X ¼ Cl, Y ¼ F 3.0116 1.5629 1.4162 �9.25 �24.60 2.5654 1.3362 96.4 1.6816 �180.22 �24.92X ¼ Cl, Y ¼ Cl 3.3002 1.5503 1.6909 �10.37 �0.79 2.8398 1.3291 97.1 1.7596 �98.74 �9.97X ¼ Br, Y ¼ F 3.0275 1.5711 2.0578 �35.74 �6.60 2.5917 1.3402 95.7 2.1086 �130.22 �34.62X ¼ Br, Y ¼ Cl 3.4018 1.5478 1.8254 �7.78 �0.26 2.8331 1.3325 97.4 1.8867 �69.62 �17.04X ¼ Br, Y ¼ Br 3.3870 1.5522 2.1956 �5.08 �1.57 2.9277 1.3302 97.8 2.2564 �44.88 �12.82

aBond length in A, bond angle in degree, frequency in cm�1, energy in kJ mol�1.bDFreq and DE represent the difference of the properties between the complexes and the monomer, DFreq is the frequency

changes of stretching vibration of XAY bond.



atom and the center of C3¼¼C4 bond (RXA*C3C4),the bond angle u(which formed by X atom, thecenter of C3¼¼C4 bond and C3 atom) becomeslarger and larger in the sequence of Y ¼ H, F, Cland Br. For the same Y atom, RXA*C3C4 becomeslarger and larger in the sequence of X ¼ H, F, Cl,and Br.

INTERACTION ENERGIES AND FREQUENCYCHANGES

The calculated interaction energies (DE) of thetitled systems are listed in Table II, which werecorrected by zero-point energy and BSSE. Com-pare complexes (a) and (b), for the same XY, theinteraction energy of complex (b) is larger thanthat of complexes (a), which means complex (b) isstable than complex (a). For the same X atom, theinteraction energies become smaller and smallerin the sequence of Y ¼ F, Cl, and Br in both com-plexes (a) and (b). For the same Y atom, the inter-action energies decrease in complexes (a) whereasit increase in complexes (b) in the sequence ofX ¼ H, F, Cl, and Br.

The changes of the XAY bond stretching vibra-tion frequency (DFreq) are also shown in Table II.It can be seen that the formation of the complexesresulted in a red-shift of XAY stretching vibration.The DFreq of complexes (a) are within the rangeof �5.08 to �182.46 cm�1, the DFreq of complexes(b) are within the range of �65.93 to �314.28cm�1. In comparison with complexes (b), the red-shift of XAY stretching vibration in complexes (a)is slight. The decreased stretching vibration fre-quency means weakening of a chemical bond,therefore, the weakening extent of XAY bond incomplexes (a) is smaller than that of in complexes(b). It also means that the interaction between XYand MECP in complexes (a) is weaker than thatof in complexes (b). For the same X/Y atom, thered-shift of XAY stretching vibration becomesslighter and slighter in the sequence of Y/X ¼ H,F, Cl, and Br.

The calculated geometries, interaction energies,and frequency changes show that that the weakinteraction indeed exists between XY and MECP.Furthermore, the interaction energies, the changesof frequency, and bond lengths have same ten-dencies. All these tendencies mean that the inter-action between XY and typical-p bonding electronpair of the C¼¼C bond of MECP is stronger thanthat of between XY and the pseudo-p bondingelectron pair associated with a CAC bond of the

cyclopropane ring. For the same X atom, the inter-actions become weaker and weaker in thesequence of Y ¼ F, Cl, and Br. For the same Yatom, it becomes weaker and weaker in com-plexes (a) while it becomes stronger and strongerin complexes (b) in the sequence of X ¼ H, F, Cl,and Br.

THE TOPOLOGICAL ANALYSES OF TOTALELECTRON DENSITY

The AIM theory has been used extensively forstudies of chemical bond [32–37]. Within theQTAIM theory, two bonded atoms in a moleculeare always accompanied by ‘‘bond path’’ linkingtheir nuclei. The point on the bond path with thelowest value of the electron density is the bondcritical point (BCP). A particular structure, inwhich the bond path links an atom to the BCP ofa bond [38], is called ‘‘conflict catastrophe’’ struc-ture. The evidence for the existence of hydrogenbond (or halogen bond) is the identification of theBCPs between the H/halogen atom and the Lewisbase [15, 16].

Figure 3 presents the molecular graphs and thecontour maps of Laplacian of complexes (a) and(b) based on the total electron density. Table IIIlists the topological characteristics at BCPs ofthese weak interaction systems.

TOPOLOGY AND ELECTRON DENSITY(qc) ATTHE BOND CRITICAL POINTS(BCPS)

By analyzing the bond paths of complexes (a),the ‘‘conflict catastrophe’’ structure is found. Thereis an interaction line between the X atom of XYand the BCP at the midpoint of C1AC2 bond ofthe cyclopropane ring, that is, in complexes (a),the BCP of C1AC2 bond is the attractor for thebond path linking X and C1AC2 bond. The elec-tron densities (qc) of the BCP are within the rangeof 0.0050–0.0160 a.u., the values does fall withinthe proposed range of 0.002–0.040 a.u. for thehydrogen bond [36, 37]. Again it has been shownthat qc is related to the bond strength. As a result,for complexes (a), the qc at BCP of the weak bondis related to RXA*C1C2. The smaller qc, the larger ofRXA*C1C2 is. A linear relationship between the qcand DRXAY was also found for the complexes (a):qc ¼ 0.00363 þ 0.50002DRXAY; the linear correla-tion coefficient R ¼ 0.9936. DRXAY is the elonga-tion of the proton-donating bond due to the weakbond formation, the larger DRXAY, the larger qc isand the weak bond is stronger. This relationship

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shows that the topological criteria [36, 37] is wellconsistent to the geometric criteria [25].

The bond path in complexes (b) is different tothat in complexes (a). In complexes (b), the weak

bond path of the bond links X atom of XY and C4atom of MECP. The electron densities (qc) of theBCP are within the range of 0.0149–0.0643 a.u.,the qc of the BCP of the halogen bond is larger

TABLE IIITopological parameters at the BCP of the weak bond in MECP. . .XY (X, Y 5 H, F, Cl, and Br)a.

Complexes (a) Complexes (b)

qc qc qc(sigma) qc(pi) d(XAC4)b d(XAC4)(sigma) d(XAC4)(pi)

X ¼ H, Y ¼ F 0.0160 0.0201 0.1782X ¼ H, Y ¼ Cl 0.0102 0.0149 0.1591X ¼ H, Y ¼ Br 0.0086 0.0138 0.1535X ¼ F, Y ¼ F 0.0069 0.0643 0.0461 0.0182 0.5458 0.4052 0.1406X ¼ Cl, Y ¼ F 0.0100 0.0312 0.0181 0.0131 0.3983 0.2959 0.1024X ¼ Cl, Y ¼ Cl 0.0060 0.0189 0.0096 0.0093 0.2633 0.1863 0.0770X ¼ Br, Y ¼ F 0.0116 0.0346 0.0207 0.0139 0.4635 0.2860 0.1775X ¼ Br, Y ¼ Cl 0.0050 0.0226 0.0128 0.0098 0.3236 0.2422 0.0814X ¼ Br, Y ¼ Br 0.0061 0.0191 0.0106 0.0085 0.2814 0.2096 0.0718

aAll values in a.u.b d is the integration of electron density over the interatomic surface of two atom.

FIGURE 3. Molecular graph (a) and electron density of Laplacian (b) of the complexes. [Color figure can be viewedin the online issue, which is available at wileyonlinelibrary.com.]



than the proposed range of 0.002–0.040 a.u. forthe hydrogen bond [36, 37], which means that thehalogen bond is stronger than the hydrogenbond. A linear relationship of the qc and DRXAY

was also found for the complexes (b): qc ¼�0.05257 þ 4.80711DRXAY; R ¼ 0.9670.

Comparing the qc at the BCP of weak bond, itcan be seen that for the same XY, the qc in com-plexes (b) is larger than that of in complexes (a).The larger the qc, the stronger of the interactionis. So it can be concluded that complexes (b) isstable than complexes (a); for the same X atom,the interaction between XY and MECP becomesweaker and weaker in the sequence of Y ¼ H, F,Cl, and Br. For the same Y atom, for complexes(a), it becomes weaker and weaker in thesequence of X ¼ H, F, Cl, and Br. For complexes(b), it becomes stronger and stronger in the conse-quence of X ¼ H, F, Cl, and Br. These conclusionsare in good agreement with the energetic andgeometric results discussed previous.

INTEGRATED PROPERTIES OF INTERATOMICSURFACE

The integration of electron density over intera-tomic surface could provide useful bonding infor-mation for the interacting atoms. The integration ofelectron density over interatomic surface of X andC4 (d(X, C4)) in complexes (b) were performed. Incomplexes (a), the bond path links X atom to thecenter of C1AC2 bond, instead of an atom, so theintegration of electron density over interatomicsurface were not performed for complexes (a).

The d(X, C4) in complexes (b) are listed in TableIII. The tendencies of d(X, C4) are similar to thoseof the qc at the BCP, energetic, and geometric dis-cussed above. An exponential relationship betweeninteratomic surface d(X, C4) and qc at BCP has beenfound, d(X, C4) ¼ A þ Bexp(�qc/C), see Figure 4.For hydrogen bond systems, A ¼ 0.1997, B ¼�0.2473, and C ¼ 0.0824, R2 ¼ 0.9999; and for halo-gen bond systems, A ¼ 0.5733, B ¼ �0.8502, and C¼ 0.0184, R2 ¼ 0.9857. As for the whole weak bondsystems, including hydrogen bond systems andhalogen bond systems, A ¼ 0.5830, B ¼ �0.9229,and C ¼ 0.0188, R2 ¼ 0.9320. Compared this rela-tionship with the bond order (n) which Bader et al.[38] defined in terms of the values of qc using thefollowing equation: n ¼ exp [A(qc � C)], it is can beseen the similar exponential function exist both inthe bond order with the qc and in the integration ofelectron density over the interatomic surface and

the qc. These relationships mean that the integra-tion of electron density over the interatomic surfaceis also a good measure of the weak bond strength,just like the qc at BCP and the bond order.

THE TOPOLOGICAL ANALYSES OF r AND pELECTRON DENSITY FOR COMPLEXES (B)

The interactions between MECP and XY (X, Y¼ H, F, Cl, and Br) in complexes (b) have both rand p component. To illustrate the detailed inter-action in MECP…XY complexes (b) (X, Y ¼ H, F,Cl, and Br), the total and separated r and p elec-tron density of complexes (b) were analyzed.

Table III also gives the qc, and d(XAC4) of totaland separated r and p electron density in com-plexes (b). The qc of total electron density iswithin the range of 0.0196–0.0643 a.u. Among it,the r electron density qc(sigma) spans the interval

FIGURE 4. The relationship between the qc of weakbond BCP and interatomic surface d(X, C4). [Colorfigure can be viewed in the online issue, which isavailable at wileyonlinelibrary.com.]

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0096–0.0461, the p electron density qc(pi) variesfrom 0.0085 to 0.0182, the ratio of qc(sigma) with qcmeans that the r interaction is stronger than the pinteraction. The d(XAC4) of total and separated rand p electron density shows the same results:the d(XAC4) of total electron density in complexes(b) in varies 0.2633 to 0.5468, the d(XAC4)(sigma) iswithin the range of 0.2096–0.4052, the d(XAC4)(pi)is within the range of 0.0814–0.1775. Therefore, incomplexes (b) the r interaction between XY andMECP are stronger than the p interaction. Inanother word, r electron density influenced thestrength of the hydrogen (halogen) bond.

Figure 5 gives the molecular graphs of totaland r electrons, and the contour maps of Lapla-cian of p electron. As shown in Figure 5, molecu-lar graphs of total electron density of hydrogenbond and halogen bond in complexes (b) are simi-lar, i.e., the H atom and the halogen atom linkto the C4 atom. The contour maps of Laplacianof the p electron density of MECP…HX andMECP…XY are similar too: both the depletion-site

of H atom and halogen atom are directed to C4atom in the C3¼¼C4 p bond. While the moleculargraphs of r electron density are different: inMECP…HX, the bond path of the hydrogen bondlinks the H of HX, and the BCP of C3AC4 bond,which is very similar to that of in MECP…XYcomplexes (a); while in MECP…XY(X is halogenatom), the X atom is link to C4 atom. As dis-cussed above, although there are some differencesthat exist in r electron density, the p electron den-sity is similar, the total electron density is similartoo. In another word, it is the direction of p elec-tron density that determined the total direction ofthe hydrogen (halogen) bond.

Conclusions

The nature of the hydrogen and halogen bondbetween MECP and XY(X, Y ¼ H, F, Cl, and Br)have been theoretically investigated. Two types of

FIGURE 5. Molecular graphs and Laplacian of r and of p electron density of MECP…XY. [Color figure can beviewed in the online issue, which is available at wileyonlinelibrary.com.]



weak interactions were compared. The analysescarried out in this work lead to the followingmain features:

1. Two seats of MECP interacted with XY arelocated: complexes (a) is the electrophilicend interact with MECP via the pseudo-pbonding electron pair associated with a CACbond of the cyclopropane ring, complexes(b) is via the typical-p bonding electron pairof the C¼¼C bond. As a whole, complexes (b)are more stable than complexes (a).

2. For the same X atom, the interactionbetween MECP and XY(X, Y ¼ H, F, Cl, andBr) becomes weaker and weaker in thesequence of Y ¼ H, F, Cl, and Br. For thesame Y atom, it becomes weaker and weakerin the sequence of X ¼ H, F, Cl, and Br forcomplexes (a), while it becomes stronger andstronger in the sequence of X ¼ H, F, Cl,and Br for complexes (b).

3. The topology structure of complex (a) and(b) are different: in complexes (a), there is aninteraction line between the X atom of XYand the BCP of C1AC2 interacting bond ofthe cyclopropane ring; In complexes (b), thebond path of weak bond links X atom of XYand C4 atom. The separated electron densityshows r electron density influenced thestrength and p electron density determinedthe direction of the hydrogen (halogen)bond.

4. The integration of electron density over theinteratomic surface is found to be a goodmeasure of the weak bond strength.

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