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Theoretical evidence for intramolecular hydrogen bonding in 7-norbornenol
Volume 74. number
CHEMICAL
3
THEORETICAL
PHYSICS
EVIDENCE FOR INTRAMOLECULAR
LETTERS
15 September
1980
HYDROGEN BONDING IN 7-NORBORNENOL
Kelp MOROKUlkfA Immure fbr ,lfofecular Scrence I%[\odaqt, Ohazahi 414 Japan
and G. WIPFF Insrrnrt de Clrrmre BPj296jR8. Recewed
67000 Srmsbourg
France
30 hfa) 1980
The stable form of 7-norbomenol, optmuzed by an SCF method, is syn-as unth an mtramolecular hydrogen bondmg to the C=C YTbond. This fcrm possesses the low OH stretchmg frequenq and small sphtnng between the C=C n and the 0 lone pur ~oruzciuon pokmttal. attnbuted to x-h> drogen bondmg
1. Introduction Intramolecular hydrogen bonding to x-electron systems has been shown to occur m many orgamc compounds [ 1,2]. One of the best “ehpenmenlal evidence” can be found m 7-norbomenol. Whereas the anti isomer shows a single OH band at 3630 cm-l, the
syn Isomer shows two OH stretchmg frequencies at 3574 and 3624 cm-l wtuch have been attnbuted to hydrogen bonded and free OH, respectively [3] The photoelectron (PE) spectra of the syn isomer show vertul Ionization potentials at 9.41 and 9.71 eV, wkch have been assigned to lomzatlon from the C=C TIsystem and from the oxygen lone pair, respectlvely, whde the ant] Isomer shows them at 9.19 and 10.01 eV. respectively. The smaller sphttmg between the two bands for the syn isomer (O-30 eV) than for the ant] Isomer (0 85 eV) has been attnbuted to the
n-hydrogen bonding m the former [4]. Despite advances m ab metro molecular orbital (MO) studies on hydrogen bondmg [s-7], no direct “theoretical evidence” supportmg the ezristence of n-hydrogen bondmg in syn-7-norbomenol or reiatmg the “experimental evidence” to n-hydrogen bonding has been presented. MO studies on several model systems have suggested that n-hydrogen bondmg IS weak [8-IO], 400
and therefore
a comparison
of stabihty
and
spectral property changes among hydrogen-bonded and non-hydrogen-bonded isomers would require a careful optunizatlon of the respective geometnes. The purpose of the present ab uutlo MO study is to optlmlze the geometry of Isomers/conformers of 7norbomenol and show that the cis conforrnauon of the syn isomer, the only form that can have a n-hydrogen bond, actually IS energetically more stable than the other forms and possesses the lowest OH stretchmg frequency, the smallest sphttmg of PE spectra and other properties that have been attnbuted to n-hydrogen bondmg. We also use some model calculations to exanune the nature of the mteractlon, the cis and trans forms of norbomalenol are also Included for companson.
2. Results The geometnes of four forms of 7-norbomenol, syn-cls (SC), syn-trans (ST), ant]-cis (AC) and anti-tram (AT), have been fully optimized with the ab initio SCF method usmg a nummal (STO-3G) basis set. The energy gra&ent and geometry opturuzatlon procedure has been used [ 1 I, 121, and a C, symmetry has been assumed m all the calculations. The optimized geometry
CHEMICAL
Volume 74~ number 3
ST
SC
AC
PHYSICS
AT
of SC, the only system ~th a possible a-hydrogen bond, is shown m fig. 1 *. Some essential charactenstics of optrmized geometries, the relatrve energy, the electron distribution, orbital energes and vibrational frequencies calculated at the optimized geometnes are summanzed 111table 1. One can make the following observations_ (1) The energy increases in the order SC < AC l
Geometnes of the other isomers are avdable request.
from KIM upon
Fig 1. Optmuzed geometry of syn-7-norbomenol.
Table 1 Opurruzed geomemes, relatwe energes, electron Istnbutlon, Conformation AE (kul/mol) RoH (A) COH (deg) populahon c= Ho 0 OH WOH
(cm_‘)
scaled
-168 a) 0 9906 103.9 6.083 0.823 8.294 0253 4220 3601
roruzahon potential (eVj Q0l-i c=c rr a) me to-
enew
is -341.49403
8 87 8 24 Eh_
15 September 1980
< AT < ST. Though we have not released a Cs constraint, we expect SC is the equilibrium geometry and ST a saddle point for the OH torsion. The anti isomer seems to have a very small torsional barrier. SC has an energy 1.7 kcal/mol lower than AC and this energy may be considered to be the hydrogen bond energy. (See section 3 for more discussion.) ST is less stable than AC and AT, presumably because of the 0 lone pan (go)-lr repulsion. (3) The OH distance m SC (O-99 1 R) is definitely larger than those (0.988 A) in the other forms. Correspondingly the overlap population of the OH bond 1s the smallest in SC, uxhcatmg that the OH bond is weaker in SC than m the other forms. The COH angle 1s also the smallest in SC. (3) In SC there is a small positive bond population nH ___c= = 0.18 X 10e2 between the OH proton and the olefmic C atom, indicating a bonding interaction. For the other forms the proton-olefmic C and the proton-aliphatic C bond population are much smaller m magmtude. (4) Atomic populatrons on the hydrogen bonding H and 0 change little among different forms. The population on the olefiiic C is largest in SC. This is contrary to a simple picture ofcharge transfer From the C=C A bond to the of orbital of OH, but is consistent with the general trend that *he electron redistnbutron is determined principally by the polarizenon effect [lo], I.e., the polarity 0-a Hf6 induces a
wbrabonal frequencies and ioruzahon potentials of 7-norbomenol
ST
SC
LETTERS
AC
1 56 O.9876 104 5 6 070 0.824 8.294 0.255 4264 3639 8.814 7 81
AT wo) O.9884 LOS.3
6 074 0.825 8.294 0.254 4254 (3630) 8.92 8.09
0.38 0987s 104.3 6.076 0.822 8.300 0.256 4265 3639 8.85 7.99
Volume 74,
negative charge on the olefuuc bond m SC, whrle m ST the oxygen lone pair induces a positive charge on the same bond. (5) The frequency WOH was estunated from the diagonal force constant for the OH bond stretching (wth the off-dsdlagonal elements neglected) and the reduced mass for OH. In a STO-3G SCF calculation, vibrational frequencies are usually overestimated by 15-30% [ 131. We have scaled W0H with the ratro 3630 (exp.)/4254 (talc ) for AC. The scaled WOH shows that in SC there IS a red-sluft of about 30 cm-l relative to AC, which IS m reasonable agreement with the experunental shrft of about 50 cm-l [3]. (6) The first two lomzation potentrals obtamed wrth Koopmans theorem lndlcate that the C=C 51orbltaI (MO 30) IS destablhzed m SC, whereas the 0 ptype lone par (MO 29) changes Intie, relative to AC. The calculated sphttmg between the two levels m SC (0.63 eV) IS smaller than that for AC (0.82 eV), a correct trend but not as large as expenmentally observed (0.30 eV and 0 85 eV) [4] _ All the “theoretlcal e$ldence” above clearly indicates that there IS an mtramolecular hydrogen bond mvolvmg an olefinic rr bond. “Eupenmental evidence”, such as the PE sphttmg and the IR shift, can really be attnbuted to hydrogen bondmg Calculations on the CIS and trans conformers of 7norbomadlenol with a geometry derived from SC and ST. respectively, show also that cis IS more stable than trans (MI?= 2 6 kcal/mol), but differing from the norbomenol case. both rrcc and oxygen lone par hlOs are more stable m the CIS than m the trans form (A = 0.32,0.19,0.19 eV for rf+, rr-, Q,, respectively) The other parameters follow the same trends as in the norbomenols; among them, the OH overlap populatlon lower m cis (0.252e) than m trans (0.258e) can be related to the two OH stretchmg frequencies observed tn the overtone regron of the IRspectrum (6928 and
7087 cm-l
[ 141).
3. Discussion We have Just given theoretlcal
evidence
that syncis
is the most stable form of 7-norbomenol.
llus can m the trans and/
result from destabdizmg Interactions or stabthzmg interactions m the SC form. Admittedly arbitrarily, one might consider the differences SC
402
15 September IYW
CHEMICAL PHYSICS LETTERS
number 3
- AC (l-7), AC - AT (0.4) and AT - ST (1.2 kcal/ mol) as the hydrogen bonding energy, the torsional energy and the Qo- n repulsron energy, respectively. In syn-7-norbornenol the bond distance between the 0 atom and the midpomt of the C=C bond, 2.89 A, 1s substantially smalier than the equihbnum distance rn the HzO-ethylene model complex, ==3.65 A and the “hydrogen bondmg energy” IS larger than the interaction energy in the model complex (0.8 kcal/mol) [9]. The norbomene skeleton forces the OH and C= C groups to mteract, and under such a condlhon, hydrogen bondmg is the most preferred. Such constramts are aiso present m norbomadlenol [ 141 and other ngd systems [IS] where intramolecular hydrogen bonding has been reported, however when these groups are not constramed to mteract, the most stable conformer may not have an intramolecular H-bond [ 161. We have performed calculations on two model H,O-ethylene complexes, SC and ST, m whrch all the geometrical parameters are taken from the STO3G optumzed SC and ST forms, respectively, of 7norbomenol, except that carbon atoms I,4 and 7 are replaced by hydrogen atoms on the same bond axes at. appropnate bond drstances. We find a repulsive mteraction m both systems, but SC IS more stable than ST (by 1.0 kcaI/mol, 3G basis set; 4.0 kcal/mol, 4-3 1G basis set). The difference arises at least m part from hydrogen bonding m SC. Indeed, the energy component analysts [S] for SC and ST model complexes with the 4-3 1G basis set (table 2) shows that SC IS preferred to ST mamly due to the electrostatic rnteraction (ES) supplemented with the charge-transfer (CT) mteractlon, m accord with the general trend observed m hydrogen bond complexes [5]. The exTable 2
Components of mteration kcal/mol) a)
enerw for model complexes (ii
Model
SC
S-l-
INT ES PL EX CT MIX
5.1 -3.3 -0.6 12.0 -2.6 -0.4
9.1 0.9 -0.7 10 6
-It.9 02
z.5 4.0 42 00 -1.4 0.7 0.5
a) A pontnz (negatwz) number dengnates destabdization (stabilization).
Volume 74, number 3
CHEMICAL
change repulsion (EX) 1s larger m SC, presumably because the hydrogen bonding H is buried more deeply in the electron cloud of the C=C R bond. Like in other hydrogen bond complexes, charge IS transferred from the electron donor ethylene to the proton donor Hz0 (0_006e, 4-3 1 G basis set) in SC, whereas the net charge transfer IS very small m ST; tlus can be related to the observation that the strength of a ST. . . HO hydrogen bond increases with the baslclty of the rr electrons [ 171. A comparison of orbltal energres in ethylene, HZ0 SC and ST, shows that, whereas ncc and 1~;~ are stabdized m SC and destabilized m ST, the o- and ptype oxygen lone pairs are destabihzed m both model complexes. Thus, stabihzation of the orbltals of the electron donor n fragment IS more characteristrc of Hbond formatlon than the destabtizarlon of the orbitals of the proton donor fragment, as seen above for norbornenol. These orbltal energy changes can be better understood as an electrostatic field effect than an orbital rmxlng effect. Whereas rrcc and ?r& are in the positive electrostatic field of H+& m SC (to be stabtied) and the negatwe field of Om6 in ST (to be destabdlzed), the oxygen lone pairs are always m the negative field of the n electrons (to be destabtized). The orbital mwng would allow rrcc (symmetric) to mix only wth the o-type QO, the energy of the former would be more conformatlon dependent, but this is not the case. The four optlrmzed geometries show common structural features concemmg the norbomene skeleton, one of the most unportant being the nonplanarity of the R system, with the olefmic C-H bonds shghtly bent endo. Thus leads to an asymmetry of the R bond which may be related to the exo stereoselectron observed m reactions of norbomene with electrophdes [IS] _Thx problem wdl be &scussed else-
PHYSICS
LEI-l-ERS
15 September I980
Numerical calculatrons have been carried out in part at the Computer Center of the Institute for Molecular Science.
References [I ] hf D. Joesten and L-J. Scbaad, Hydrogen bonding (Dekker, New York, 1974).
121 G C. Rmentel and A.L. McClellan, The hydrogen bond t31 141 151 IsI
171
PI 191 tw t111 1121 iI31
(Freeman, New York, 1970). R I;. Bly and R S. Bly. J. Org. Cbem. 28 (1965) 3165. R-S. Brown, Can. J. Cbem. 54 (1976) 3206. K. hforokuma. Accounts Chem Res. 10 (i977) 294. P. Schuster, in. The hydrogen bond, Vol. 1. eds. P. Schuster. G. Zundel and C. Sandorfy (North-Holland, Amsterdam, 1976) p. 25. P-A. Kollman, m Modern theoretical chemistry. Vol. 4, ed. H-F. Schaefer III (Plenum Press, New York. 1978) p. 109. K hforokuma, J. Cbem Phys. 5.5 (1971) 1236. J De! Bene. Cbem. Phys. Letters 24 (197-f) 203. S Yamabe and K hforokuma, J. Am. Chem. Sot. 97 (1975)4458. A. Komomicla, K. fshlda, K. hforokuma. R. Ditchfield and hf. Conrad. Chem. Phys. Letters 45 (1977) 595. K. hforokuma, S. Kate, K. Kitaura, f. Ohmineand S. Sakai, fMSPACK program, unpubbsbed. P- Pulay, III- hfodem theorehcal chemistry. Vol. 4. ed. H F. Schaefer III (Plenum Press, New York, 1978) p_
153. [I41 S Wiistem and C. Clrdonneau, I. Am. Chem. Sot. 82 (1960) 2084.
1151 R S. Brown and R-L. hfarcmko.J. Am. Chem. Sot 99
iW
(1977) 6500; hf. Takasuka and H. Tanida, J. Cbem. Sot Perkin IL (1980) 486, and references tberem. R J. _4braham and J.M. Bakke. Tetrahedron 34 (1978) 2947.
t171 A-W. Baker and A-T. shulgin, J. Am. Chem Sot 80 (1958) 5358. H. Fujlmoto and K. Fukui, Sot. 98 (1976) 4054.
1181 S. fna@,
J. Am. Cbem.
where.
403
CHEMICAL
3
THEORETICAL
PHYSICS
EVIDENCE FOR INTRAMOLECULAR
LETTERS
15 September
1980
HYDROGEN BONDING IN 7-NORBORNENOL
Kelp MOROKUlkfA Immure fbr ,lfofecular Scrence I%[\odaqt, Ohazahi 414 Japan
and G. WIPFF Insrrnrt de Clrrmre BPj296jR8. Recewed
67000 Srmsbourg
France
30 hfa) 1980
The stable form of 7-norbomenol, optmuzed by an SCF method, is syn-as unth an mtramolecular hydrogen bondmg to the C=C YTbond. This fcrm possesses the low OH stretchmg frequenq and small sphtnng between the C=C n and the 0 lone pur ~oruzciuon pokmttal. attnbuted to x-h> drogen bondmg
1. Introduction Intramolecular hydrogen bonding to x-electron systems has been shown to occur m many orgamc compounds [ 1,2]. One of the best “ehpenmenlal evidence” can be found m 7-norbomenol. Whereas the anti isomer shows a single OH band at 3630 cm-l, the
syn Isomer shows two OH stretchmg frequencies at 3574 and 3624 cm-l wtuch have been attnbuted to hydrogen bonded and free OH, respectively [3] The photoelectron (PE) spectra of the syn isomer show vertul Ionization potentials at 9.41 and 9.71 eV, wkch have been assigned to lomzatlon from the C=C TIsystem and from the oxygen lone pair, respectlvely, whde the ant] Isomer shows them at 9.19 and 10.01 eV. respectively. The smaller sphttmg between the two bands for the syn isomer (O-30 eV) than for the ant] Isomer (0 85 eV) has been attnbuted to the
n-hydrogen bonding m the former [4]. Despite advances m ab metro molecular orbital (MO) studies on hydrogen bondmg [s-7], no direct “theoretical evidence” supportmg the ezristence of n-hydrogen bondmg in syn-7-norbomenol or reiatmg the “experimental evidence” to n-hydrogen bonding has been presented. MO studies on several model systems have suggested that n-hydrogen bondmg IS weak [8-IO], 400
and therefore
a comparison
of stabihty
and
spectral property changes among hydrogen-bonded and non-hydrogen-bonded isomers would require a careful optunizatlon of the respective geometnes. The purpose of the present ab uutlo MO study is to optlmlze the geometry of Isomers/conformers of 7norbomenol and show that the cis conforrnauon of the syn isomer, the only form that can have a n-hydrogen bond, actually IS energetically more stable than the other forms and possesses the lowest OH stretchmg frequency, the smallest sphttmg of PE spectra and other properties that have been attnbuted to n-hydrogen bondmg. We also use some model calculations to exanune the nature of the mteractlon, the cis and trans forms of norbomalenol are also Included for companson.
2. Results The geometnes of four forms of 7-norbomenol, syn-cls (SC), syn-trans (ST), ant]-cis (AC) and anti-tram (AT), have been fully optimized with the ab initio SCF method usmg a nummal (STO-3G) basis set. The energy gra&ent and geometry opturuzatlon procedure has been used [ 1 I, 121, and a C, symmetry has been assumed m all the calculations. The optimized geometry
CHEMICAL
Volume 74~ number 3
ST
SC
AC
PHYSICS
AT
of SC, the only system ~th a possible a-hydrogen bond, is shown m fig. 1 *. Some essential charactenstics of optrmized geometries, the relatrve energy, the electron distribution, orbital energes and vibrational frequencies calculated at the optimized geometnes are summanzed 111table 1. One can make the following observations_ (1) The energy increases in the order SC < AC l
Geometnes of the other isomers are avdable request.
from KIM upon
Fig 1. Optmuzed geometry of syn-7-norbomenol.
Table 1 Opurruzed geomemes, relatwe energes, electron Istnbutlon, Conformation AE (kul/mol) RoH (A) COH (deg) populahon c= Ho 0 OH WOH
(cm_‘)
scaled
-168 a) 0 9906 103.9 6.083 0.823 8.294 0253 4220 3601
roruzahon potential (eVj Q0l-i c=c rr a) me to-
enew
is -341.49403
8 87 8 24 Eh_
15 September 1980
< AT < ST. Though we have not released a Cs constraint, we expect SC is the equilibrium geometry and ST a saddle point for the OH torsion. The anti isomer seems to have a very small torsional barrier. SC has an energy 1.7 kcal/mol lower than AC and this energy may be considered to be the hydrogen bond energy. (See section 3 for more discussion.) ST is less stable than AC and AT, presumably because of the 0 lone pan (go)-lr repulsion. (3) The OH distance m SC (O-99 1 R) is definitely larger than those (0.988 A) in the other forms. Correspondingly the overlap population of the OH bond 1s the smallest in SC, uxhcatmg that the OH bond is weaker in SC than m the other forms. The COH angle 1s also the smallest in SC. (3) In SC there is a small positive bond population nH ___c= = 0.18 X 10e2 between the OH proton and the olefmic C atom, indicating a bonding interaction. For the other forms the proton-olefmic C and the proton-aliphatic C bond population are much smaller m magmtude. (4) Atomic populatrons on the hydrogen bonding H and 0 change little among different forms. The population on the olefiiic C is largest in SC. This is contrary to a simple picture ofcharge transfer From the C=C A bond to the of orbital of OH, but is consistent with the general trend that *he electron redistnbutron is determined principally by the polarizenon effect [lo], I.e., the polarity 0-a Hf6 induces a
wbrabonal frequencies and ioruzahon potentials of 7-norbomenol
ST
SC
LETTERS
AC
1 56 O.9876 104 5 6 070 0.824 8.294 0.255 4264 3639 8.814 7 81
AT wo) O.9884 LOS.3
6 074 0.825 8.294 0.254 4254 (3630) 8.92 8.09
0.38 0987s 104.3 6.076 0.822 8.300 0.256 4265 3639 8.85 7.99
Volume 74,
negative charge on the olefuuc bond m SC, whrle m ST the oxygen lone pair induces a positive charge on the same bond. (5) The frequency WOH was estunated from the diagonal force constant for the OH bond stretching (wth the off-dsdlagonal elements neglected) and the reduced mass for OH. In a STO-3G SCF calculation, vibrational frequencies are usually overestimated by 15-30% [ 131. We have scaled W0H with the ratro 3630 (exp.)/4254 (talc ) for AC. The scaled WOH shows that in SC there IS a red-sluft of about 30 cm-l relative to AC, which IS m reasonable agreement with the experunental shrft of about 50 cm-l [3]. (6) The first two lomzation potentrals obtamed wrth Koopmans theorem lndlcate that the C=C 51orbltaI (MO 30) IS destablhzed m SC, whereas the 0 ptype lone par (MO 29) changes Intie, relative to AC. The calculated sphttmg between the two levels m SC (0.63 eV) IS smaller than that for AC (0.82 eV), a correct trend but not as large as expenmentally observed (0.30 eV and 0 85 eV) [4] _ All the “theoretlcal e$ldence” above clearly indicates that there IS an mtramolecular hydrogen bond mvolvmg an olefinic rr bond. “Eupenmental evidence”, such as the PE sphttmg and the IR shift, can really be attnbuted to hydrogen bondmg Calculations on the CIS and trans conformers of 7norbomadlenol with a geometry derived from SC and ST. respectively, show also that cis IS more stable than trans (MI?= 2 6 kcal/mol), but differing from the norbomenol case. both rrcc and oxygen lone par hlOs are more stable m the CIS than m the trans form (A = 0.32,0.19,0.19 eV for rf+, rr-, Q,, respectively) The other parameters follow the same trends as in the norbomenols; among them, the OH overlap populatlon lower m cis (0.252e) than m trans (0.258e) can be related to the two OH stretchmg frequencies observed tn the overtone regron of the IRspectrum (6928 and
7087 cm-l
[ 141).
3. Discussion We have Just given theoretlcal
evidence
that syncis
is the most stable form of 7-norbomenol.
llus can m the trans and/
result from destabdizmg Interactions or stabthzmg interactions m the SC form. Admittedly arbitrarily, one might consider the differences SC
402
15 September IYW
CHEMICAL PHYSICS LETTERS
number 3
- AC (l-7), AC - AT (0.4) and AT - ST (1.2 kcal/ mol) as the hydrogen bonding energy, the torsional energy and the Qo- n repulsron energy, respectively. In syn-7-norbornenol the bond distance between the 0 atom and the midpomt of the C=C bond, 2.89 A, 1s substantially smalier than the equihbnum distance rn the HzO-ethylene model complex, ==3.65 A and the “hydrogen bondmg energy” IS larger than the interaction energy in the model complex (0.8 kcal/mol) [9]. The norbomene skeleton forces the OH and C= C groups to mteract, and under such a condlhon, hydrogen bondmg is the most preferred. Such constramts are aiso present m norbomadlenol [ 141 and other ngd systems [IS] where intramolecular hydrogen bonding has been reported, however when these groups are not constramed to mteract, the most stable conformer may not have an intramolecular H-bond [ 161. We have performed calculations on two model H,O-ethylene complexes, SC and ST, m whrch all the geometrical parameters are taken from the STO3G optumzed SC and ST forms, respectively, of 7norbomenol, except that carbon atoms I,4 and 7 are replaced by hydrogen atoms on the same bond axes at. appropnate bond drstances. We find a repulsive mteraction m both systems, but SC IS more stable than ST (by 1.0 kcaI/mol, 3G basis set; 4.0 kcal/mol, 4-3 1G basis set). The difference arises at least m part from hydrogen bonding m SC. Indeed, the energy component analysts [S] for SC and ST model complexes with the 4-3 1G basis set (table 2) shows that SC IS preferred to ST mamly due to the electrostatic rnteraction (ES) supplemented with the charge-transfer (CT) mteractlon, m accord with the general trend observed m hydrogen bond complexes [5]. The exTable 2
Components of mteration kcal/mol) a)
enerw for model complexes (ii
Model
SC
S-l-
INT ES PL EX CT MIX
5.1 -3.3 -0.6 12.0 -2.6 -0.4
9.1 0.9 -0.7 10 6
-It.9 02
z.5 4.0 42 00 -1.4 0.7 0.5
a) A pontnz (negatwz) number dengnates destabdization (stabilization).
Volume 74, number 3
CHEMICAL
change repulsion (EX) 1s larger m SC, presumably because the hydrogen bonding H is buried more deeply in the electron cloud of the C=C R bond. Like in other hydrogen bond complexes, charge IS transferred from the electron donor ethylene to the proton donor Hz0 (0_006e, 4-3 1 G basis set) in SC, whereas the net charge transfer IS very small m ST; tlus can be related to the observation that the strength of a ST. . . HO hydrogen bond increases with the baslclty of the rr electrons [ 171. A comparison of orbltal energres in ethylene, HZ0 SC and ST, shows that, whereas ncc and 1~;~ are stabdized m SC and destabilized m ST, the o- and ptype oxygen lone pairs are destabihzed m both model complexes. Thus, stabihzation of the orbltals of the electron donor n fragment IS more characteristrc of Hbond formatlon than the destabtizarlon of the orbitals of the proton donor fragment, as seen above for norbornenol. These orbltal energy changes can be better understood as an electrostatic field effect than an orbital rmxlng effect. Whereas rrcc and ?r& are in the positive electrostatic field of H+& m SC (to be stabtied) and the negatwe field of Om6 in ST (to be destabdlzed), the oxygen lone pairs are always m the negative field of the n electrons (to be destabtized). The orbital mwng would allow rrcc (symmetric) to mix only wth the o-type QO, the energy of the former would be more conformatlon dependent, but this is not the case. The four optlrmzed geometries show common structural features concemmg the norbomene skeleton, one of the most unportant being the nonplanarity of the R system, with the olefmic C-H bonds shghtly bent endo. Thus leads to an asymmetry of the R bond which may be related to the exo stereoselectron observed m reactions of norbomene with electrophdes [IS] _Thx problem wdl be &scussed else-
PHYSICS
LEI-l-ERS
15 September I980
Numerical calculatrons have been carried out in part at the Computer Center of the Institute for Molecular Science.
References [I ] hf D. Joesten and L-J. Scbaad, Hydrogen bonding (Dekker, New York, 1974).
121 G C. Rmentel and A.L. McClellan, The hydrogen bond t31 141 151 IsI
171
PI 191 tw t111 1121 iI31
(Freeman, New York, 1970). R I;. Bly and R S. Bly. J. Org. Cbem. 28 (1965) 3165. R-S. Brown, Can. J. Cbem. 54 (1976) 3206. K. hforokuma. Accounts Chem Res. 10 (i977) 294. P. Schuster, in. The hydrogen bond, Vol. 1. eds. P. Schuster. G. Zundel and C. Sandorfy (North-Holland, Amsterdam, 1976) p. 25. P-A. Kollman, m Modern theoretical chemistry. Vol. 4, ed. H-F. Schaefer III (Plenum Press, New York. 1978) p. 109. K hforokuma, J. Cbem Phys. 5.5 (1971) 1236. J De! Bene. Cbem. Phys. Letters 24 (197-f) 203. S Yamabe and K hforokuma, J. Am. Chem. Sot. 97 (1975)4458. A. Komomicla, K. fshlda, K. hforokuma. R. Ditchfield and hf. Conrad. Chem. Phys. Letters 45 (1977) 595. K. hforokuma, S. Kate, K. Kitaura, f. Ohmineand S. Sakai, fMSPACK program, unpubbsbed. P- Pulay, III- hfodem theorehcal chemistry. Vol. 4. ed. H F. Schaefer III (Plenum Press, New York, 1978) p_
153. [I41 S Wiistem and C. Clrdonneau, I. Am. Chem. Sot. 82 (1960) 2084.
1151 R S. Brown and R-L. hfarcmko.J. Am. Chem. Sot 99
iW
(1977) 6500; hf. Takasuka and H. Tanida, J. Cbem. Sot Perkin IL (1980) 486, and references tberem. R J. _4braham and J.M. Bakke. Tetrahedron 34 (1978) 2947.
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1181 S. fna@,
J. Am. Cbem.
where.
403