Exciplex and charge-transfer complex fluorescence in the inter- and

J . Phys. Chem. 1992, 96, 5796-5800

5796 750

In all cases the blue shift is increased by deuteriation. On deuteriation the strength of the hydrogen bond is increased, implying that something else is responsible for the change in u(CN) shifts. One can assume that the answer lies in the antibonding character of the nitrogen free electron pair which is influenced by the substituents. If one normalizes the blue shifts on the strength of the hydrogen bonds (Av(CN) divided by vI or Av(HF)) one can give the following order of X for decreasing antibonding influence of the nitrogen free electron pair in the CN bond: F > Br > C1 > CH3 > H > SFS CF3 > CN. (iv) The free energy of complex formation cannot be correlated with the strength of the hydrogen bond because the energy of the XCN submolecule is altered. This point was discussed earlier on the example OC.-HF and CO--HF.'4

1

450

References and Notes (1) Andrews, L. J. Phys. Chem. 1984,88,2940. (2) Andrews, L.; Hunt, R. D. J. Phys. Chem. 1988, 92, 81. (3) Hunt, R. D.; Andrews, L. J. Phys. Chem. 1987, 91,5594. (4) Hunt, R. D.; Andrews, L. Inorg. Chem. 1987, 26, 3051. (5) Hunt, R. D.; Andrews, L. J. Phys. Chem. 1987, 91, 2751. (6) Davis, S. D.; Andrews, L. J. Mol. Spectrosc. 1985, 1 1 1 , 219. (7) Johnson, G. L.; Andrews, L. J. Phys. Chem. 1983,87, 1852. (8) For an overview, see for example: Bevan, J. K. NATO ASI Ser., Ser. C 1987, no. 212, 149. (9) Thomas, R. K. Proc. R.SOC.London 1971, 325, 133. (10) Georgiou, A. S.,Legon, A. C.; Millen, D. J. Proc. R. Soc. London A 1980, 370, 257. (1 1) Wang, F. M.; Iqbal, K.; Kraft, H.G.; Luckstead, M.; Eue,W. C.; Bevan, J. W. Can. J. Chem. 1982, 60, 1969. (12) Wofford, B. A,; Jackson, M. W.; Lieb, S. G.; Bevan, J. W. J. Chem. Phys. 1988,89, 2775. (13) Wofford, B. A.; Ram, R. S.; Quinonez, A.; Bevan, J. W.; Olson, W. B.; Lafferty, W. J. Chem. Phys. Lett. 1988, 152, 299. (14) Schatte, G.; Willner, H.; Hoge, D.; Kn6zinger, E.; Schrems, 0. J. Phys. Chem. 1989, 93, 6025. (IS) Fawcett, F. S.;Lipscomb, R. D. J. Am. Chem. Soc. 1964,86,2576. (16) Brauer, G., Ed. Handbuch der Praparativen Anorganischen Chemie, Vol. 2, 3rd ed.; Friedrich Enke: Stuttgart, 1978; p 630. (17) Brauer, G., Ed. Handbuch der Praparativen Anorganischen Chemie, Vol. 2, 3rd ed.; Friedrich Enke: Stuttgart, 1978; p 632. (18) Brauer, G., Ed. Handbuch der Praparativen Anorganischen Chemie, Vol. 2, 3rd ed.; Friedrich Enke: Stuttgart, 1978; p 628. (19) Jacobs, J.; McGrady, G. S.; Willner, H.; Christen, D.; Oberhammer, H.; Zylka, P. J. Mol. Struct. 1991, 245, 275. (20) Edgell, W. F.; Potter, R. M. J. Chem. Phys. 1956, 24, 80. (21) Burger, H; Pawelke, G. J. Chem. Soc., Chem. Commun. 1988, 105. (22) Andrews, L.; Johnson, G. L. J. Phys. Chem. 1984, 88, 425. (23) Schurvell, H. F.; Faniran, J. A. J. Mol. Spectrosc. 1970, 33, 436.

Exciplex and Charge-Transfer Complex Fluorescence in the Inter- and Intramolecular Jet-Cooled EDA Systems Noriyuki Kizu and'Michiya Itoh* Faculty of Pharmaceutical Sciences, Kanazawa University, Takara-machi, Kanazawa 920, Japan (Received: January 16, 1992; In Final Form: March 9, 1992) The exciplex fluorescence was observed in the excitation of the van der Waals complex between 9,lO-dicyanoanthracene and naphthalene in supersonicexpansion,where no signifcant vibrational energy dependence was observed in the transformation of the complex to the exciplex. In the jet-cooled intramolecular EDA system of (9,10dicyano-2-anthryl)(CHz)3( I-naphthyl), very broad excitation and diffuse red-shifted fluorescence spectra were observed. The red-shifted fluorescenceof the jet-cooled intramolecular EDA system seems attributable to the emission of the intramolecularly interacted charge-transfer state formed in the ground state. In inter- and intramolecular systems of 9-cyano-IO-methylanthracene/naphthalene and (9-cyano-10anthryl)(CHz)s(2-naphthyl), neither vdW complex nor charge-transfer interaction was detected in both the ground and excited states.

Inter- and Intramolecular Jet-Cooled EDA Systems Franck-Condon (FC) excited state of the complex was ascribed to the resonance state rather than the charge-transfer (CT) state. Further, Tramer and his co-worker~,'*~ and also Haas and his g r o u p also reported the jet-cooled exciplex formations in similar EDA systems. On the other hand, in the early stage of research of the supersonicjet, Russell and Levy9reported the fluorescence of the chargetransfer (CT) complex between tetracyanoethylene (TCNE) and p-xylene in supersonic expansion and very broad and diffuse fluorescence excitation and dispersed spectra which are insensitive to the frequency of the exciting light. Since the FC excited state of the CT complex may have mostly CT character, the above-mentioned exciplex generated from the excited vdW complex may be clearly distinguishable from the CT complex. In the solution and also in the static vapor phase, the exciplex formation is well-known to be the consquence of an encounter collision between the ground- and excited-state molecules followed by the relaxation to the CT state, though these molecules are strictly repulsive in the ground state. Zewd and his cuworkers'oJ1 reported the intramolecular exciplex formation in the jet cooled 1-(9-anthryl)-3-(4-NJV-dimethylanilino)propane,and a remarkable exvibrational energy dependence which is corresponding to an activation energy of the intramolecular exciplex formation. The activation energy was ascribed to an energy for the intramolecular collisional dynamics between the ground- and excited-state moieties. The feature of this intramolecular exciplex formation in supersonic expansion is completely different from the exciplex formations in the jet-cooled vdW complexes, mentioned above. In this circumstance, inter- and intramolecular exciplex formations in the same EDA systems were examined in supersonic expansion. The fluorescence excitation spectrum of the jet-cooled mixed system of 9,lO-dicyanoanthracene (DCA) and naphthalene exhibits red-shifted and considerably broad vibrationalstructures suggesting the vdW complex formation. The dispersed exciplex fluorescence (& 460-470 nm) was observed in the excitations of these vdW complex bands. In the intramolecular EDA system of 1-(9,10-dicyano-2-anthryl)-3-(1naphthy1)propane (DCAN), the fluorescence excitation spectra indicate the considerably strong intramolecular CT interaction in the ground state. The large Stokes-shifted fluorescence observed in the jet-cooled DCAN was ascribed to the ground-state CT complex fluorescence,which is different from the other jet-cooled intramolecularEDA system in respect of their formation dynamics, mentioned above. In inter- and intramolecular systems of 9cyano-1 0-methylanthracene/naphthalene and (9-cyano- 10anthryl)(CH2)3(2-naphthyl), however, neither vdW complex nor CT interaction was detected both in the ground and excited states.

Experimental Section The experimental setup and procedures of pulsed supersonic free jet are completely similar to those described in the previous papers.I2*l4In the jet-cooled mixed system of DCA and naphthalene, naphthalene vapor in a heated sample bottle was mixed with He, and the mixture was allowed to pass over heated DCA crystals in the sample reservoir. The measurements of laser-ind u d fluorescence and decay times were performed in the same method as reported previously. The synthesis and purification of intramolecular EDA compounds (DCAN) were reported in the previous paper^.'^-^^ Naphthalene (zone refined, Tokyo Kasei Co.) and naphthalene-ds (Merck Sharp & Dohme of Canada Ltd.) were used without further purification. Results and Discussion The fluorescence excitation spectrum of bare 9,lO-dicyanoanthracene exhibits considerably well-resolved vibrational structures starting from an origin band at 25 137 cm-' (397.82 nm)." Figures 1, parts a and b, shows the fluorescence excitation spectra of the mixed free jet of DCA and naphthalene system monitored in the 430- and 480-nm regions, respectively. The spectrum monitored at 480 nm exhibits broad bands approximately 4.8 nm red-shifted (313 cm-I) from origin and each vibronic bands of vibrational structures of bare DCA, while the 430-nm fluorescence excitation spectrum is completely similar to that of bare DCA.

The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5797

h

bl

Figure 1. (a) Fluorescence excitation spectrum of bare DCA in a supersonic free jet monitored at 430 nm. (b) Fluortscence excitation spectrum of the DCA and naphthalene mixed system monitored at 480 nm.

400 Wnm 500 Figure 2. Dispersed fluorescence spectra of the jet-cooled DCA/ naphthalene system obtained by excitations of the origin and vibronic

bands of the vdW complex. The broad bands increase in intensity with increasing vapor pressure of naphthalene. Since no additional band appeared even in rather high vapor pressure of naphthalene in the He carrier gas, the bands approximately 4.8 nm red-shifted from the origin and each vibronic band of DCA may be attributable to the 1:l vdW complex between DCA and naphthalene in supersonic expansion. Excitation of these relatively broad bands attributable to the vdW complex results in the dispersed fluorescence spectra shown in Figure 2. Since the Stokes shift of these dispersed fluorescence spectra is approximately 4500-5000 cm-'from the excitation wavelength, the spectra may be attributable to the exciplex between DCA and naphthalene generated from the vdW complex. Further, a very weak fluorescence with a short decay time attributable to the vdW complex between these compounds was detected at 400-450 nm by a time-resolved fluorescence as will be mentioned later. The nonpolar solution of these compounds was reported to exhibit the exciplex fluorescence in rather high concentration at room temperature, while the solution exhibits the similar large Stokes shifted fluorescence (A,, 490 nm) attributable to the CT complex generated in the ground state at low temperature, as reported in previous papers.I6 The theoretical considerations on the CT state suggests that the excited state of the ground-state EDA complex may have a CT structure identical with that of the excited-state EDA complex (exciplex) generated only in the excited state, irrespective of their formation dynamics, though their excited Franck-Condon states may be different from each other. In this respect, the experimental evidence of the identical fluorescent state of the ground-state EDA complex with the exciplex was reported in the DCA and naphthalene system by Itoh and MimuraI6 and in the 1,2,4,5-tetracyanobenzeneand pxylene system by Gaweda and Prochorow.'s In general, taking account of the considerable blue shift of dispersed fluorescence of the jet-cooled molecule compared with fluorescence in solution, the broad fluorescence spectra (A, 470 nm) observed in the

5798

The Journal of Physical Chemistry, Vol. 96, No. 14, 1992

Kim and Itoh

\ -

430

nm

?e" J

-

403.2 n m

4 8 0 nm

1 400

Figure 3. Fluorescence excitation spectra of jet-cooled DCAN monitored at 430 and 480 nm (a sample reservoir temperature was approximately 160 "C).

excitations of vdW complex shown in Figure 2 may be ascribed to the emission from the CT state, which is essentially identical with those of the exciplex and the CT complex observed in nonpolar solution.16 It is noteworthy that three different excited species exhibit the large Stokes-shifted fluorescence from an identical fluorescent state, though formation dynamics of these fluorescent states are completely different each other. In the exciplex formation from the vdW complex between 1-cyanonaphthalene and related compounds, and triethylamine,12 a remarkable excess vibrational energy dependence of the exciplex formation was reported, as mentioned above. However, the fluorescence excitation and the exciplex fluorescence of the vdW complex between DCA and naphthalene do not seem to depend on the excitation energy. The fact indicates that the transformation of the vdW complex to the exciplex takes place even in their lowest vibrational energy level of the S1state. Fluorescence decay profiles were determined in the excitations of several vibrational energy levels of the vdW complex. The decay curves detected at 420 and 480 nm consist of double-exponentialdecay times of 2.7 and 68 ns. The short decay component is predominant (95%) in the former decay curve (420 nm fluorescence), while the long one is dominant (75%) in the latter decay one (480 nm fluorescence). The former and latter decay times are ascribed to the resonance fluorescence of the vdW complex and the exciplex, respectively. However, no rise time was detected in the exciplex fluorescence because of considerable overlapping of the resonance fluorescence. In the vibronic band excitations of the complex, these decay times of the resonance fluorescence and the exciplex are almost independent of the excitation energy level. On the other hand, the bandwidth of the vdW complex in the fluorescence excitation spectra (Figure 1) is remarkably broad compared with those of the complex between 1-CNN and methyl-substituted 1-CNN, and triethylamine reported in the previous papers. In methyl-substituted 1-CNN and TEA systems,12the vdW complex whose excitation spectra exhibit band broadening not only by the intermolecular vibration but also by the intramolecular vibrational redistribution (IVR) indicates no significant excess energy dependence of the exciplex formation from vdW complex. In the DCAlnaphthalene system reported here and other anthracene/ aromatic amine systems,s IVR within the component molecules of the vdW complex seems to enhance the broadening of the bandwidths of excitation spectra and consequently to enhance the level crossing between the LE state of the complex and the CT state (exciplex) even in the lower vibrational energy level. The vdW complex and exciplex formations were also examined in jet-cooled DCA and naphthalene-& However, no significant deuterium isotope effect was observed in the exciplex formation dynamics except for a 17-19 cm-l red shift of the complex bands in the excitation spectra. Figure 3 shows the fluorescence excitation spectra of the intramolecular EDA system 1-(9,10-dicyano-2-anthryl)-3-(1naphthy1)propane (DCAN) in supersonic expansion. The excitation spectrum of the long-wavelength fluorescence indicates a

I h/nm

394.4 n m

1

450

I 500

550

Figure 4. Dispersed fluorescence spectra of jet-cooled DCAN in the excitation of the intramolecularly ground-state complex bands.

very broad and completely structureless spectrum, while that of the 420-nm fluorescence is considerably resolved on some broad background. The excitation of the structureless band of this jet-cooled DCAN leads to the considerably large Stokes shifted fluorescence as shown in Figure 4. The compound exhibits intramolecular exciplex fluorescence in nonpolar solution (A, 480-530 nm) at room temperature, while the compound in the same solution shows the fluorescence due to the CT complex fluorescence at low temperature, reported in the previous papers, Therefore, the broad structureless fluorescence observed in jetcooled DCAN (Figure 4) may be attributable to the fluorescence of the intramolecular ground-state complex between EDA moieties in DCAN. Here, it is unlikely that the very broad fluorescence and excitation spectra are attributable to the intermolecular interaction between two DCAN molecules because of very low vapor pressure of this compound. If the excited Franck-Condon state of the complex may be mostly the locally excited and resonant state of the complex and the level crossing from the resonant state to the CT state takes place followed by the fluorescent relaxation, the large Stokes-shifted fluorescence may be really ascribed to the exciplex. However, the Franck-Condon excited state of the complex formed in the ground state has a major character of the CT state, so that the broad fluorescence may be attributable to the fluorescence of the CT complex. Taking account of the completely structureless fluorescence observed here, the spectra shown in Figure 4 may be ascribed to the intramolecular CT complex fluorescence formed in the ground state, though the Stokes shift of the fluorescence is approximately 400W500 cm-'. The other intramolecular EDA system, l-(9-cyano-l0anthry1)-3-(1-naphthyl)propane (9-CAN), exhibits fluorescence excitation spectra with well-resolved vibrational structures in supersonic expansion in the detection wavelength of 410-480 nm. Figure 5 shows the fluorescence excitation spectra of this jet-oooled compound in comparison with that of 9-cyano-10-methylanthracene. These spectra do not seem to indicate any intramolecular interaction between two EDA moieties of 9-CAN in both the ground and excited states. As mentioned in the Introduction, if some excess vibrational energy is required for the exciplex formation, the fluorescence excitation spectra should be examined in the shorter wavelength region up to 370 nm (corresponding to excess vibrational energy, 1650 cm-'). However, these excitation spectra and also dispersed fluorescence spectra were almost independent of excitation wavelength up to 370 nm. Further, the fluorescence excitation spectra of jet-cooled 9cyano-10-methylanthracene mixed with naphthalene indicate neither exciplex nor vdW complex formations between them. These facts suggest no significant electronic interaction between 9-cyanoanthraceneand naphthalene in either the ground or excited state. The completely different feature between DCAN and 9-CAN may be attributable to the different electron affinity between 9,lO-dicyanoanthryl (DCA) and 9-cyanoanthryl (CA) moieties in both the ground and excited states. Kebarle and ChouwdhuryIgestimated electron affities (EA) of many aromatic compounds by the ion-molecule equilibrium technique and determined that that of 9-CA (1.27 eV) is 0.67 eV larger than that

Inter- and Intramolecular Jet-Cooled EDA Systems

n-

R-

Figure 6. Schematic illustration of potential energy curves of the EDA interaction; the FC excited state of the CT complex (a) is ascribed to the CT state, while that of the vdW complex (b) is ascribed to the LE state.

of anthracene (0.60 eV); therefore that of DCA moiety may be approximately 1.5-1.8 eV. The different feature in the groundstate interaction between DCAN and 9-CAN seems attributable to these different electron affinities (EA). Since the electron affinity of the SIexcited state of DCA and CA moieties may roughly correspond to energy levels of the highest molecular orbitals of these moieties, probably smaller ionization potential of CA does not seem enough for the excited-state CT interaction with the naphthalene moiety. As mentioned in the Introduction, the anthracene/Nfl-dimethylanilineintermolecular system forms the vdW complex in supersonic expansion leading to the exciplex formation in In the intramolecular system of 1-(9anthryl)-3-(pN,N-dimethylanilino)propane, exciplex formation takes place through intramolecular collision between excited anthryl and ground-state Nfldimethylanilino moieties. This fact means that the vdW interaction between EDA moieties in the ground state is not required for the exciplex formation in this intramolecular EDA system. In the jetcooled intermolecualr EDA system, however, exciplex fluorescence has never been detected without vdW complex formation prior to the photoexcitation. This is because intermolecular collision between the ground- and excited-state molecules leading to the exciplex formation may be impossible in the jet-cooled isolated molecular condition. As mentioned in the Introduction, Russell and Levy9 reported the fluorescence of the CT complex between TCNE and pxylene in supersonic expansion. Both the excitation and emission spectra of this jet-cooled CT complex are broad and diffuse, and the fluorescence spectrum is insensitive to the frequency of the exciting light. The diffuseness in the spectrum and its insensitivity t o the excess vibrational energy were attributed to relaxation via the rapid inter- and intramolecular redistribution of the excess vibrational

The Journal of Physical Chemistry, Vol. 96, No.14, 1992 5799

energy into a large density of isoenergetic vibrational levels of the CT state. The most important feature of fluorescence and excitation spectra in these jet-cooled EDA systems is that the Franck-Condon excited state of TCNElpxylene system may be mostly CT character, while that of the vdW complex such as 1-cyanonaphthaleneand triethylamine may belong to the locally excited (LE) or resonance state of the complex.1~2~12 In the latter case of the vdW complex, the rapid level crossing of the rmnance state to the CT state may take place followed by the exciplex fluorescence. In the jet-cooled intramolecular EDA system of 1-(9anthryl)-3-(4-Nfl-dimethylanilino)propane, the intramolecular collision between two EDA moieties is essential for the exciplex and no evidence for the ground-state interaction between the two moieties was suggested. In the intramolecular EDA system of 1-(9,10-dicyanc+2-anthryl)-3-(1-naphthy1)propane (DCAN) reported here, the fluorescence excitation spectra indicate the considerably strong intramolecular CT interaction in both the ground and excited states in the supersonic expansion. The long-wavelength fluorescence observed in this jetcooled compound (Figure 4), which is similar to the exciplex observed in nonpolar hydrocarbon solution at room temperature and also to the CT complex fluorescence at 77 K,I6 is completely different from the other jet-cooled intramolecular EDA system in respect of their formation dynamics.lO*" The equilibrium configurations of the CT state in DCAN may be very different from those of the intermolecular van der Waals states of DCA/naphthalene. In the intramolecular CT configurations, the CT state may be lower in the energy level than the LE resonance state of the complex, as shown in schematic illustration of Figure 6. Therefore, the FC excited state of this jet-cooled DCAN may be ascribed to the CT state composed of a distribution of vibrational levels high in the potential The absorption to the vibrationally congested levels causes broadening of the fluorescence excitation spectrum and insensitivity of the spectrum to excitation frequency.

References and Notes ( 1 ) Saigusa, H.; Itoh, M. Chem. Phys. Lett. 1984, IM,391; J. Chem. Phys. 1984, 81, 5682. (2) Saigusa, H.; Itoh, M.; Baba, H.; Hanazaki, I. J. Chem. Phys. 1987, 86, 2528. (3) Castella, M.; Rwhorow, J.; Tramer, A. J . Chem. Phys. 1984,81, 251 1 . (4) Castella, M.; Tramer, A.; Piuui, F. Chem. Phys. Lett. 1986,129. 105; Chem. Phys. Lett. 1986,129, 112. (5) Castella, M.; Millie, P.; Piuzzi, F.; Caillet, J.; Langlet. J.; Claverie, P.; Tramer, A. J . Phys. Chem. 1989, 93, 3949. (6) Anner, 0.;Haas, Y . Chem. Phys. Lett. 1985, 119, 199. (7) Anner, 0.;Haas, Y . J. Phys. Chem. 1986, 90,4298. (8) Anner, 0.;Haas, Y . J. Am. Chem. Soc. 1988,110,1416 and references therein. (9) Russell, T. D.; Levy, D. H. J . Phys. Chem. 1982, 86, 2718. (10) Felker, P. M.; Syage, J. A.; Lambert, W. R.; Zewail, A. H. Chem. Phys. Lett. 1982, 92, 1 .

J. Phys. Chem. 1992, 96, 5800-5803

5800

(1 l ) Syage, J. A.; Felker, P. M.; Zewail, A. H. J . Chem. Phys. 1984,81, 2233. (12) Itoh, M.; Sasaki, M. J. Phys. Chem. 1990, 94, 6544. (13) Itoh, M.; Hayashi, A. J . Phys. Chem. 1989, 93, 7789. (14) Itoh, M.; Takamatsu, M.; Kizu.; Fujiwara, Y . J . Phys. Chem. 1991, 95, 9682. (15) Itoh, M.; Mimura, T.; Usui, H.; Okamoto, T. J. Am. Chem. SOC. 1973, 95, 4388.

(16) (a) Itoh, M. J . Am. Chem. SOC.1974, 96, 7390. (b) Itoh, M.; Mimura, T. Chem. Phys. Lett. 1974, 24, 551. (17) Hirayama, S.; Tanaka, F.; Shobatake, K. Chem. Phys. Leu. 1987, 140, 447. (18) Gaweda, M.; Prwhorow, J. Chem. Phys. Lett. 1975, 30, 155. (19) Kebarle, P.; Chouwdhury, S. Chem. Reu. 1987, 87, 513. (20) Castella, M.; Millie, P.; Piuzzi, P.; Caillet, J.; Langlet, J.; Claverie, P.; Tramer, A. J . Phys. Chem. 1989, 93, 3941.

Bond Dissociation Energies of H,NX Compounds. Comparison with CH,X, HOX, and FX Compounds Kenneth B. Wiberg Department of Chemistry, Yale University, New Haven, Connecticut 0651 1 (Received: October 16, 1991)

The bond dissociation energies (BDE) of H2NNH2,HONH,, and FNH2 have been calculated using Pople's G1 procedure, and the known BDE for hydrazine is well reproduced. The BDEs of MeX, H2NX, HOX, and FX derivatives, where X = Me, H2N, HO, and F, are compared, and the differences are related to changes in hybridization, internal Coulombic stabilization, and lone pair-lone pair repulsion.

In another investigation, we made an indirect estimate of the bond dissociation energies of hydrazine, hydroxylamine, and fluoroamine and found that all were close to 65 kcal/mol.' This is in agreement with the known dissociation energy of hydrazine, 66 kcal/mol? The similarity in values for these compounds stands in contrast to the change in dissociation energies for the corresponding methyl derivatives: methylamine, 84.1; methanol, 90.2; and methyl fluoride, 108.2 kcal/mol.2 As a first step in trying to understand the differences in the two series, we have carried out direct calculations of the dissociation energies. In order to have appropriate comparison data, we have examined the following series: CH3X, H2NX, HOX, and FX where X = CH3, NH2, HO, and F. The structures were calculated at the MP2/6-3 lG* theoretical level (Table I), giving geometries which generally agreed satisfactorilywith the experimental~ a l u e s . ~ Energies were calculated at the MP4/6-3 1l++G**, QCISD(T)/631 l++G**, and QCISD(T)/6-31 l++G(Zdf,p) theoretical level^.^ These data are summarized in Table 11. The radicals formed by the dissociation processes also were studied, and their data are included in the table. The calculation of bond dissociation energies is known to present problems. One good solution has been provided by Pople et al. in their G1 procedure.' This involves calculation of the energy at the QCISD(T)/6-3 11+G(2df,p) level, either directly or via a set of calculations. The energies are then corrected for higher level terms (hlc) and for the zero-point energies. The latter are estimated using the HF/6-31GS vibrational frequencies that are scaled by the factor 0.8934.5 The use of the 6-31 1++G(2df,p) basii set in our work, which includes diffuse functions at hydrogens, leads to slightly different higher level correction terms.6 The full calculation of Eo = E(QCISD(T)/6-31 l++G(Zdf,p)) + Ehlc+ EZpEis given in the last column of Table 11. The dissociation energies calculated at the various levels are summarized in Table 111. The MP4 and QCISD(T) values obtained using the 6-31l++G** basis set are essentially the same and are somewhat too small as compared to the experimental values.2 The MP2/6-31G* dissociation energies are close to the experimental value. All sets of estimated Do are linearly related to the observed values, as shown in Figure 1 for the MP2 data. Here, the relationship between the MP2 data, after correcting for zero-point energy changes, and the experimental data was BDE(MP2) = 0.8 + 0.988BDE(obs) The rms error was only 1.4 kcal/mol. The good agreement is,

of course, a result of cancellation of errors. It will be interesting to see whether this agreement extends to other saturated molecules. The use of the 6-3 1 l++G(Zdf,p) basis that includes two sets of d functions and one set off functions on the non-hydrogenated atoms leads to a considerable decrease in calculated energies and an improvement in the dissociation energies. The inclusion of Pople's higher level corrections further improves the agreement between calculated and observed values, leading to an rms error of 0.7 kcal/mol. The calculations confirm our previous estimate of the dissociation energies of hydroxylamine and fluoroamine and show that they are indeed about the same as that for hydrazine. It now remains to examine the reasons for the differences between the MeX, H2NX, HOX, and FX series (Figure 2). The bond dissociation energies in the MeX series appear to be dominated by chargetransfer effects. After a decrease on going from ethane to methylamine, there is a steady increase in BDE on going from methylamine to methanol and methyl fluoride. The decrease in BDE on going from ethane to methylamine is probably a hybridization effect. Whereas the methyl group is sp3 hybridizul, the amine group uses less s character in its bonds in order to use as much s character as possible to stabilize its lone pair. This results in a C-N-H bond path angle of 106.4O.' It is known that the percent s character has a dramatic effect on BDE's, so that in the series ethane, ethylene, and acetylene the C-H BDE's are 98, 110, and 131 kcal/mol, respectively.8 The increase in BDE from methylamine to methyl fluoride is probably related to the increasing ionic character of the bonds that results from the increasing difference in electronegativity. This is reflected in the charges calculated for the groups in these molecules via numerical integration of the charge density from the MP2 wave functions using appropriately defined atomic volumes (Table IV).9 Pauling showed that ionic character increases bond strengths as a result of the added Coulombic stabilization.1° It is generally thought that the low dissociation energy for fluorine is due to the repulsion of the lone pairs attached to the atoms." It may be an important factor with the H2NX compounds since the nitrogen has a lone pair and could become even more important with the HOX compounds. Repulsive interactions usually lead to changes in geometry, and so we have examined the M-X bond lengths (Figure 3). Here, for consistency, the calculated bond lengths were used. The same conclusion would have been reached using the experimental ~ a l u e s . ~

0022-365419212096-5800$03.00/0 0 1992 American Chemical Society