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Journal of Molecular Structure: THEOCHEM 820 (2007) 80–84

A theoretical investigation on the potential energy surface ofCN2H rotation of the encapsulated 1-bicyclo[2.2.1]heptyldiazirine

Xiuli Wang, Zuoyin Yang, Ju Wang, Jingchang Zhang, Weiliang Cao *

State Key Laboratory of Chemical Resource Engineering, Faculty of Science, Beijing University of Chemical Technology, Beijing 100029, PR China

Received 1 May 2007; received in revised form 31 May 2007; accepted 12 June 2007Available online 21 June 2007

Abstract

The potential energy surface (PES) of CN2H rotation of the encapsulated 1-bicyclo[2.2.1]heptyldiazirine (BHD) inside a molecularcontainer: Cram’s hemicarcerand (CH) was explored using two different DFT involved ONIOM methods: B3LYP/6-31G**//ONI-OM(B3LYP/6-31G*: AM1) and B971/6-31G**//ONIOM(B971/6-31G*: AM1). The free-state PES of CN2H rotation was also calcu-lated, respectively by B3LYP/6-31G**//B3LYP/6-31G* and B971/6-31G**//B971/6-31G* methods for comparison. The findings inthis study have shown that the PES profiles differ from each other notably in the two states. In the encapsulated state the rotation barriercorresponding to the free-state conversion with the largest rotation barrier increases by about 2 kcal/mol, which has exceeded the largestrotation barrier in the free-state. The conformational preference behavior towards certain BHD isomers, which might be in better con-formational compatibility with the container, has been demonstrated.� 2007 Elsevier B.V. All rights reserved.

Keywords: Potential energy surface; ONIOM methods; Cram’s hemicarcerand; Rotation barrier

1. Introduction

The successful generation of the first container molecule‘hemicarcerand’ by Donald J. Cram has opened up a fasci-nating area in chemistry [1–3]. It consists of two half-bowlresorcinarene units, which are connected together by fourlinking groups, and could incarcerate a single guest mole-cule permanently. Following this, many other containermolecules have been synthesized using either covalent bondor weak intermolecular forces, e.g. hydrogen bond andmetal–ligand interactions [4–11]. The investigation of con-tainer molecular complexes and single molecule encapsula-tion has provided a novel way to understand someimportant chemical and biological phenomena such asselective separation [8,12], stabilization of reactive interme-diates [13–16], catalysis [6,7,10] and new type of isomerism[17], and it has attracted much attention in the past years.However, the most frequently used experimental method:

0166-1280/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.theochem.2007.06.013

* Corresponding author. Tel.: +86 10 64444919; fax: +86 10 64434898.E-mail address: caowl@mail.buct.edu.cn (W. Cao).

1H NMR could be mainly employed to demonstrate theformation of the complex. The already reported computa-tional studies on such kind of complexes have mostlyinvolved force field calculations (MM2, MM3, AMBER)[18–22], which cannot give agreeable results withoutincluding explicit electronic interactions. Therefore despitenumerous efforts towards the synthesis and investigation ofvarious types of functional container molecules and suchcomplexes, their intrinsic properties still have not been wellestablished. Encouragingly, studies in this area employingquantum chemical methods are increasingly emerging andmuch useful information was brought about due to the fastdevelopment in computer software and hardware nowa-days [23–29].

Recently, P. Roach et al. has reported the encapsulationof 1-bicyclo[2.2.1]heptyldiazirine (BHD, Fig. 1) in the cav-ity of Cram’s hemicarcerand (CH, Fig. 1) to study theroom temperature stabilization of anti-Bredt high strainolefins: bicyclo[2.2.2]oct-1-ene and bicyclo[3.2.1]oct-1-enewhich were generated by thermolysis or photolysis ofBHD in CH [30]. The formation of the complex CHÆBHD

Fig. 1. Fully optimized geometrical structures of the free-state BHD(DC7–C4–C8–H22 = �180�) and CHÆBHD complex (DC7–C4–C8–H22 = �175�)by B3LYP/6–31 G* and ONIOM(B3LYP/6-31G*: AM1), respectively (Hatoms on CHÆBHD were removed for clarity).

X. Wang et al. / Journal of Molecular Structure: THEOCHEM 820 (2007) 80–84 81

was demonstrated by 1H NMR spectra. However, the con-formational variations of CH and BHD after encapsula-tion, and the influences of the container’s restricted innerphase on the potential energy surface (PES) of CN2H rota-tion remain unclear yet. Herein we focus on studying theseissues with quantum chemical methods to gain someinsights. The results obtained are anticipated to be helpfulto the design of functional container molecules and tobroaden the views in the container’s modulation role inthe conformation of inner-phase molecule, which mightbe meaningful to regiochemistry in bimolecular systemsas well.

2. Computational methods

Semiempirical molecular orbit method AM1 [31] hasbeen testified to show good agreement between the calcu-lated and experimental distances in this type of host [25].Hence, it was adopted to optimize the host geometry inconsideration of the fact that eighteen geometrical struc-tures need to be optimized in the PES scan procedure tothis sizable complex CHÆBHD. To reduce computationalefforts, eight –(CH2)4CH3 groups on the CH were replacedwith hydrogen atoms, since a previously report at the AM1level suggested the host shell distortion from changing thesubstituents is negligible [25]. Geometries of the free-stateBHD and CH molecules were optimized firstly byB3LYP [32,33] and B971 [34] with 6-31G* basis set, andAM1 method, respectively. The initial model of CHÆBHDwas constructed by moving the optimized free-state BHDinto CH manually, with the head group CN2H of BHDlocated inside one polar bowl-like cavity according to therelevant report [19]. In view of the overestimation of longrange interaction with pure AM1 method, the geometryof CHÆBHD was fully optimized by two hybrid ONIOM[35–37] methods: ONIOM(B3LYP/6-31G*: AM1) andONIOM(B971/6-31G*: AM1) without any symmetricalconstraints to relieve this problem. Vibration frequencycalculations were carried out to confirm that the found

stationary points were true minima. Relaxed PES scan cal-culations were conducted to investigate the CN2H rotationof free-state BHD with B3LYP/6-31G* and B971/6-31G*

methods, while for the encapsulated complex CHÆBHD,ONIOM(B3LYP/6-31G*: AM1) and ONIOM(B971/6-31G*: AM1) methods were employed. In all the ONIOMcalculations, BHD molecule was treated in the high levellayer and CH molecule was in the low level layer. In orderto further reduce the disadvantage resulting from the par-ticipation of AM1 method in the optimization stage andmake the PES results more reliable, single point energiesof the scanned structures for the free-state BHD and theencapsulated complex were evaluated using B3LYP/6-31G** and B971/6-31G** methods. All the calculationswere carried out with GAUSSIAN 03 suite of programs[38]. Close interatomic contact distances in some confor-mations of CHÆBHD were monitored using Materials Stu-

dio software package [39].

3. Results and discussion

3.1. Conformational variations of the guest and host

molecules after encapsulation

The optimized structures of the free-state BHD with thedihedral angle DC7–C4–C8–H22 = �180� and the complexCHÆBHD with DC7–C4–C8–H22 = �175� are displayed inFig. 1. As the figure displays, the guest prefers to adoptan orientation with the diazirine group located inside thepolar bowl-like cavity. Such orientation is in agreementwith the results obtained from the 1H NMR spectra andforce field calculations [18,19]. To the encapsulated BHD,it is found that most of the bond lengths of C–C becomeslightly shortened by 0.1–0.2 pm and the bond lengths ofC–H were elongated by 0.1–0.3 pm compared with thefree-state BHD. To the bond angles, only hC7–C4–C8’s vari-ation value is larger than 0.3�, whereas over 50% of hC–C–Hsand hH–C–Hs’ variation is in the range of 0.3–0.8�, indicat-ing that the encapsulation could influence the position ofthe stretched atom C8 and the hydrogen atoms slightlymore strongly than the framework carbon atoms. The anal-ysis of dihedral angles shows that the change in angledegrees of most DC–C–C–Cs, DC–C–C–Hs or DH–C–C–Hs isabout 0.3–1.4�, while angle degrees of the dihedrals includ-ing atom N9, N10 and H22 vary relatively more obviouslywith 7.1–8.7� differences. This finding can be explainedwith the fact that the encapsulated BHD molecule nolonger adopts the most stable conformation as that in thefree-state, but favors a conformation with the CN2H groupslightly rotated around the C4–C8 bond due to the influ-ences imposed by the container.

The conformational variation analysis of CH afterencapsulation is based on the comparison between its opti-mized free-state geometry and the host’s geometry of theoptimized CHÆBHD with DC7–C4–C8–H22 = �175�. Thegeometries of CH of CHÆBHD optimized by ONI-OM(B3LYP/6-31G*: AM1) and ONIOM(B971/6-31G*:

-150 -100 -50 0 50 100 150-0.5

0.0

0.5

1.0

1.5

2.0

Dihedral angle DC7-C4-C8-H22 (o)

Rel

ativ

e en

ergy

(kca

l/mol

)

ab

Fig. 2. Calculated PES for CN2H rotation of free-state BHD with B3LYP/6-31G**//B3LYP/6-31G* (a) and B971/6-31G**//B971/6-31G* (b) methods.

82 X. Wang et al. / Journal of Molecular Structure: THEOCHEM 820 (2007) 80–84

AM1) were compared by overlapping them together andthe obtained RMSD value is 0.0026, suggesting that thegeometries optimized by these two methods are almostidentical. However, the RMSD values between the opti-mized free-state CH and the host of CHÆBHD optimizedby ONIOM(B3LYP/6-31G*: AM1), and ONIOM(B971/6-31G*: AM1) are 0.9802 and 0.9795, respectively. Thenegligible difference between the hosts from these two dif-ferent optimized methods and the large RMSD differencebetween the free-state CH and the host of CHÆBHD makeus believe that it is appropriate to use any host geometryfrom the two methods to describe its conformationalchanges after encapsulation. Here the geometry optimizedby ONIOM(B3LYP/6-31G*: AM1) is adopted. One majordistinction observed directly is that the conformation ofone of the two boat-shaped –O–(CH2)4–O– linking bridgesstretches to chair-shaped to accommodate the guest BHD.This results in the extension of the distance between thetwo oxygen atoms on the linking bridge from 4.4 to5.5 A. The distances for the other three pairs of bridge oxy-gen atoms have changed from 4.0 to 4.8 A, 5.3 to 5.8 A,and 6.1 to 6.1 A, respectively, and such results indicate thatthe linking bridges have been lengthened slightly when CHencapsulates the guest molecule. The molecular volume ofCH in the two states was computed and the values are 2916and 2943 A3, respectively, which correspond to the free-state CH and the one encapsulating BHD. As expected,volume expansion of CH, which is about 27 A3 shown bycomputational data, has taken place after encapsulation.The volume expansion rate of CH is approximately justone percent, and no obvious size increase or decrease inits cross or longitudinal direction was observed.

3.2. Analysis of CN2H rotation of BHD in the free and

encapsulated states

When optimizing the free-state BHD, several conforma-tions with different orientations of head group CN2H wereobtained. This observation was obviously due to the singlebond internal rotation of C4–C8, and the conformationsobtained should correspond to certain local minima onthe PES path. Relaxed PES scan calculations were there-fore carried out to clarify the energy profile of these rota-tion isomers with B3LYP/6-31G* and B971/6-31G*

methods. The dihedral angle DC7–C4–C8–H22 is set as thescan variable, which is one of the main geometrical param-eters to the rotation; the PES scan calculations begin withone of the free optimized geometries with DC7–C4–C8–H22 =�180�. The step size and number of steps are 20� and 18�,respectively. The energies of the eighteen geometries foundalong the PES path were then further calculated byB3LYP/6-31G** and B971/6-31G** methods, respectively.The results of the PES calculations for free-state BHDare displayed in Fig. 2. It can be seen from Fig. 2 that thereare three relatively stable conformations for free BHD,with the corresponding DC7–C4–C8–H22s being about

�180�, �50� and 60�, respectively. The energy of the con-formation with DC7–C4–C8–H22 = 60� is slightly lower thanthat with DC7–C4–C8–H22 = �180� or �50� by about0.1 kcal/mol from both B3LYP/6-31G**//B3LYP/6-31G*

and B971/6-31G**//B971/6-31G* calculations. In consider-ation of this negligible energy difference, these three confor-mations can be regarded as isoenergetic isomers, whichmight be the predominant conformations and exist withthe same probability. Both methods have shown that dur-ing the 360� rotation period of DC7–C4–C8–H22 the energyundergoes three uphill or downhill conversions with thebarriers being about 1.0, 1.7 and 1.3 kcal/mol in sequence.

In the PES study of the CN2H rotation of the encapsu-lated BHD, we firstly chose one of its free optimized geom-etries with DC7–C4–C8–H22 = �180� and placed it into CHoptimized with AM1 method, and then fully optimized thiscomplex. PES scan calculation starting from this optimizedgeometry was then conducted using the same procedure asthat of the free-state calculation. The calculated results arepresented in Fig. 3. It can be noticed that the results fromB3LYP/6-31G**//ONIOM(B3LYP/6-31G*: AM1) andB971/6-31G**//ONIOM(B971/6-31G*: AM1) do not agreewith each other as well as that of the free-state, especially inthe region from DC7–C4–C8–H22 = �75� to 145�, whereas theoverall changing trend of the energy curves is very similarto each other. The rotation process in the encapsulatedstate surpasses four energy peaks in all, and the rotationbarriers are about 0.8, 3.8, 1.8 and 1.3 kcal/mol in sequencefrom B3LYP/6-31G**//B3LYP/6-31G*; the correspondingvalues are about 0.8, 4.2, 0.5 and 2.3 kcal/mol by B971/6-31G**//B971/6-31G*. Therefore despite the differences inthe relative energy in certain section, both methods predictconsistent rotation barriers except the conversion fromDC7–C4–C8–H22 = 60� to 185� (�175�).

Table 1Interatomic distances (d, in A) falling into the defined close contacts ofthree conformations for the guest BHD in CHÆBHD complex

BHDDC7–C4–C8–

H22 = �175�

BHD DC7–C4–C8–

H22 = �75�

BHD DC7–C4–C8–

H22 = �35�

dH22. . .C92 2.648 dN9. . .C90 2.948 dC7. . .H182 2.630dC7. . .H182 2.709 dN9. . .C92 2.852 dH20. . .O108 2.513dH11. . .C26 2.698 dN9. . .H194 2.583 dH20. . .H182 2.142dH20. . .C113 2.670 dH22. . .C67 2.666 dH12. . .C116 2.562dH20. . .H182 2.170 dC7. . .H182 2.656 dH12. . .C117 2.714dH21. . .H190 2.196 dH11. . .O60 2.553 dH12. . .H187 2.196dH12. . .C116 2.591 dH21. . .H182 2.148 dH12. . .H186 2.113dH12. . .H183 2.181 dH12. . .C116 2.579 dH12. . .H183 2.131dH12. . .H186 2.121 dH12. . .H183 2.189 dH13. . .O59 2.374dH13. . .C47 2.684 dH12. . .H187 2.250 dH13. . .O123 2.229dH13. . .O59 2.386 dH12. . .H186 2.130 dH15. . .H194 2.180dH13. . .O123 2.445 dH13. . .O59 2.360 dC6. . .H148 2.681dH15. . .O123 2.479 dH13. . .O123 2.422 dH17. . .C47 2.666dH15. . .H198 2.242 dH17. . .C47 2.687 dH17. . .C65 2.677dC6. . .H148 2.678 dH17. . .C45 2.653 dH18. . .C67 2.617dH18. . .C67 2.672 dH18. . .C67 2.623 dH18. . .H153 2.193dH18. . .H148 2.253 dH18. . .H148 2.175 dH18. . .H149 2.221dH18. . .H153 2.089 dH18. . .H149 2.244dH19. . .H192 2.137 dH18. . .H153 2.104

dH19. . .H192 2.183

-150 -100 -50 0 50 100 150

0

1

2

3

4

5

Dihedral angle DC7-C4-C8-H22 (o)

Rel

ativ

e en

ergy

(kca

l/mol

) a b

Fig. 3. Calculated PES for CN2H rotation of encapsulation complexCHÆBHD by B3LYP/6-31G**//B3LYP/6-31G* (a) and B971/6-31G**//B971/6-31G* (b) methods.

X. Wang et al. / Journal of Molecular Structure: THEOCHEM 820 (2007) 80–84 83

Comparing the energy curves in the two states, a notableobservation is that the energy of the complex ascends muchquickly and the increasing amplitude is larger than the freestate along PES scan path, indicating that the rotation ofthe single bond C4–C8 of the encapsulated BHD mightbecome more difficult probably due to the steric effect fromthe container. This point is well supported by the increasedrotation barrier. The most difficult conformational conver-sion to the encapsulated BHD is between the structureswith DC7–C4–C8–H22 = �135� and �40�, and the predictedrotation barrier is about 4.0 kcal/mol, which is muchhigher than that in the free state. Fig. 3 shows that thereare three relatively stable conformations to the encapsu-lated BHD, with the corresponding DC7–C4–C8–H22s beingabout �175�, �40� and 55�, respectively. The point ataround �135� is considered to be an ‘‘inflexion point’’ onthe global energy profile as the corresponding structure isalmost isoenergetic with its anterior counterpart. Amongthese relative stable isomers, the most stable one is the con-formation with DC7–C4–C8–H22 = �175�, which was calcu-lated coherently by both methods. Compared with themost stable isomer, the subsequent relative stable isomersare higher in energy by about 0.8 and 1.2 kcal/mol, and3.5 and 1.5 kcal/mol, respectively with B3LYP/6-31G**//B3LYP/6-31G* and B971/6-31G**//B971/6-31G*. Theresults in Fig. 3 have also suggested that the two methodsagree well with each other in predicting the geometrieslocated on energy minima or maxima, while they havedivergence in the calculating the relative energy of thestructures, especially to the structure with DC7–C4–C8–H22 =�40�. In comparison of the relative energy results in Figs. 2and 3, it is found that the most stable geometry is no longerthe same one with DC7–C4–C8–H22 = 60� as in the free state,but is the geometry with DC7–C4–C8–H22 = �175�. Thisstructure differs by 5� compared with the corresponding

relative stable geometry with DC7–C4–C8–H22 = �180� forthe free-state BHD. The other two relative stable isomerswith DC7–C4–C8–H22 = �50� and 60� in the free-state shiftto positions with DC7–C4–C8–H22 = �40� and 55� whenBHD is encapsulated. Similarly the rotational transitionstates located on local high-points are also not in corre-spondence with each other in the two states, especially tothe transition structures in the highest barrier conversion,that is, DC7–C4–C8–H22 = 0� (free-state) vs. �75� (encapsu-lated). These conformational variations might be due tothe influence from the electronic and steric effects of thecontainer’s constrained inner phase, which has interruptedthe rotation movement of CN2H and altered the conforma-tions of rotation isomers slightly. In addition, Fig. 2shows that the free-state PES curve has basically goodsymmetry, whereas the symmetrical property is severelydestroyed after encapsulation (Fig. 3). The proposedexplanation for this result is that the symmetry in thestructure of the complex is lower than that in the free-stateBHD because of the unsymmetrical twisted conformationof CH.

To understand deeply the potential reasons of the rela-tively obvious differences in the energy profiles of the twostates along the PES path, the interatomic close contactsbetween BHD and CH of some CHÆBHD conformationswere monitored. The scaled sum of VDW radii optionwas adjusted to a value of 0.94 to acquire the close contactswith moderate intensity and number. The results (Table 1)show that the numbers of interatomic close contactsbetween the head group CN2H and CH of the conforma-tions with DC7–C4–C8–H22 = �175�, �75� and �35� are 1,4 and 0, respectively, and the corresponding numbersbetween the framework of BHD and CH are 18, 16 and 17.

84 X. Wang et al. / Journal of Molecular Structure: THEOCHEM 820 (2007) 80–84

It can be seen that the conformation with DC7–C4–C8–H22 =�75� has more close contacts between CN2H and CH incomparison with the conformations with DC7–C4–C8–H22 =�175� and �35�, while these between the framework ofBHD and CH remain almost equal. Thus the head groupCN2H probably possesses relatively stronger close contactwith CH when DC7–C4–C8–H22 changes to �75� along withthe rotation of CN2H inside CH, resulting in the energyat this point much higher than others. Based on these find-ings, we propose that it might be the differences in the inter-atomic close contacts between the head group CN2H andCH that alters the energy sequence for various conforma-tions along the PES path when compared with the freestate.

4. Conclusions

Theoretical investigations by two DFT involved ONI-OM calculations indicate that some conformationalchanges have occurred to the guest and host moleculesafter encapsulation. Comparative PES scan results to theCN2H rotation of BHD in the two states have shown thatin the encapsulated state the rotation barrier correspondingto the free-state conversion with the largest rotation barrierincreases by about 2 kcal/mol, which has exceeded the larg-est rotation barrier in the free state. The conformations ofthe relatively stable BHD isomers are not in consistencybetween the two states, that is to say, a conformation ofBHD that is stable in the free-state might become no longerstable when it is encapsulated in CH. Therefore it is sug-gested that the container could display some conforma-tional preference behavior towards certain guest isomers,which might be in better conformational compatibility withthe container. This finding might be valuable in the poten-tial utilization of some container molecules to constraincertain specific conformations.

Acknowledgements

We are grateful to the Special Research Fund for theDoctoral Program of Higher Education (20040010008)and the Scientific Research Fund of Beijing University ofChemical Technology (QN0411), People’s Republic of Chi-na, for financial support.

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