J. Phys. Chem. 1991, 95, 8517-8520
8517
harmonic, the intermolecular potential is in the form of a dipole-dipole interaction, and modes on different monomers are coupled in a painvise fashion, a particularly simple form emerges for the calculation of these second-order energies. Although we have applied this method to relatively small systems in the present study, the real power of this ansatz lies in its ability to treat medium and large systems in a facile manner. Our results indicate (33) that first-order effects will usually predominate over second-order effects for small dimeric systems. It is proposed, however, that and the second-order induction energy is given by this situation could be reversed when either the monomers are large or when higher order clusters are considered. We conclude that the effect of vibrational dispersion upon the stability of a van der Waals complex will always be small compared with other effects. The effect upon vibrational spectra, however, can be relatively large. In this regard, it appears that there is no corAlternative methods for obtaining the shift in frequency due to relation between the shift induced from the mean-field interactions the influence of permanent fields have been e m p l ~ y e d . ~By ~,~~ and the shift induced by vibrational dispersion interactions. We incorporatingthe effects of monomer electrical anharmonicity (i.e., anticipate that the inclusion of the effects of vibrational dispersion nonzero dipole second derivatives), first-order electrostatic perwill assist in the interpretation of IR spectra of van der Waals turbations must be in~luded.~'Additionally, one can treat the complexes with a view toward constructing more realistic intercoupling of a given mode to the permanent field of its partner molecular potentials. Moreover, theoretical predictions of mothrough the cubic term of that mode.52 nomer spectral shifts can be applied toward the problem of in situ diagnostics of aggregations in a variety of environments. 5. Conclusions The extension of London's second-order perturbational treatAcknowledgment. We thank Dr.Rick Trebino, Dr.Celeste ment of electronic dispersion to the vibrational case offers a M. Rohlfing, and Dr.Larry A. Rahn for inspiring discussions and straightforward method to evaluate intermolecular vibrational valuable suggestions. The work was performed under the auspices correlation. Under the assumptions that monomer vibrations are of the Division of Chemical Sciences, Office of Basic Energy Sciences, U. S.Department of Energy. Registry No. C02, 124-38-9; C2H4,74-85-1; HCN,74-90-8. (52) Morawitz, H.; Eisenthal, K. B. J . Chem. Phys. 1971, 55, 887.
only excitations on B. In contrast, the perturbing states have excitations on both A and B for each term in the dispersion expansion (q5 ) . After modifying eq 9 to reflect an induction rather than dispersion interaction, the perturbation operator now appears as
Conformational Effects on Charge-Transfer Properties in Selected 9,lO-Disubstituted Anthracene Derivatives: Ground- and Excited-State Dipole Moments S. Muralidharan: Hemant K. Sinha, and Keith Yaks* Department of Chemistry, University of Toronto, Toronto, Canada M5S IAI (Received: February 15, 1991)
The ground- and excited-state dipole moments of three 9,lO-disubstitutedanthracene derivatives have been determined by the method of electrochromism on absorption spectra. The ground-state dipole moments were calculated by semiempirical methods and compared with the experimentally obtained values. The extent of charge transfer on electronic excitation for the compounds studied is correlated with the torsional angle of the donor group. Comparisons of the excited-state dipole moments of benzene and anthracene derivatives were made and the results explained in terms of the difference in their molecular orbital diagrams.
1. Introduction Conformational change of the dimethylamino group in the excited state is the key concept of the proposed twisted intramolecular charge-transfer (TICT) model, used to explain the dual emission exhibited by 4-(dimethy1amino)benzonitrile (l).' Biphenyl and substituted biphenyls? binaphthyl, and bianthracene3 are also known to undergo change in conformation on electronic excitation. One key piece of experimental evidence to support the TICT hypothesis has come from the study of molecules that have ortho methyl group(s), such as 3-methyl-4-(dimethylamin0)benzonitrile (2) and 3,5-dimethyl-4-(dimethylamino)benzonitrile (3). In these molecules the steric repulsion by the ortho-substituted methyl group(s) twists the donor group away from the plane of the benzene ring, in the ground state.' The photophysical properties of the naphthalene analogue of 1 have already been examined? We recently extended the photophysical study to the anthracene analogue of 1, 9-cyano-IO-(dimethyl'Present address: Department of Molecular Biology. and Pharmacology, Box 8103. Washington University School of Medicine, St. Louis, MO 631 10.
amino)anthracene (4).13a To explore the conformational effects on the photophysical properties, the study was extended to 9(1) Grabowski, 2.R.; Rotkiewicz, K.; Siemiarczuk, A.; Cowley, D. J.; Baumann, W. Noun J . Chime., 1979, 3,443. (2) Takei, Y.; Yamaguchi, T.; Osamura, Y.; Fuke, K.; Kaya, K. J . Phys. Chem. 1988, 92, 577. (3) Mataga, N.; Yao, H.; Okada, T.; Rettig, W. J. Phys. Chem. 1989, 93, 3383 and references cited therin. (4) (a) Lippert, E.; Ayuk, A. A.; Rettig, W.; Wermuth, G.J . Phorochem. 1981, 17, 237. (b) Ayuk, A. A. J . Mol. Srrucr. 1982, 84, 169. (5) For a recent review, see: Suppan, P. J . Phorochem. Phorobiol. ( A ) 1990, 50, 293. (6) (a) Liptay, W.; Czekalla, J. Z . Narurforsch. 1960, ISa, 1072. (b) Liptay, W.; Czekalla, J. Ber. Bunsenges. Phys. Chem. 1961, 65, 721. (c) Labhart, H. Chimia 1961, 15, 20. (7) Brittinger, C.; Maiti, A. K.; Baumann, W.; Detzer, N. 2.Narurforsch. 1990, 45a, 883 and references cited therin. (8) Sinha, H. K.; Thompson, P. C. P.; Yates, K. Can. J . Chem. 1990,68, 1507. (9) Sinha, H. K.; Muralidharan, S.; Yates, K., unpublished results. (10) Grabowski, 2.R.; Dobowski, J. Pure Appl. Chem. 1983, 55, 245. (1 1) Batter, C. J. F. Theory of Elecrric Polarizarion; Elsevier: Amsterdam, 1973; Vols. 1 and 2.
0022-365419112095-8517$02.50/00 1991 American Chemical Society
8518 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991
0 CN
I
I
CN
CN
2
3
H,
Muralidharan et al.
/CH3 N
I
cyano- 10-(methy1amino)anthracene (5) and 9-amino- 10-cyanoanthracene (6), which have smaller torsional angles for the donor group, compared to compound 4. The conventional method for determining the excited-state dipole moment ( p c ) of a molecule is by the method of solvatochromi~m.~ Due to the easy experimental procedure involved, this method has found widespread applicati~n.~ A more elaborate method to determine pe is that of electrochromism,first developed by Liptay, Czekalla, and Labhart.6 This method, applied to fluorescence spectra, has been used by Baumann and co-workers to obtain the p, of various benzene derivatives.' The p, obtained by electrochromism on fluorescence spectra is for the vibrationally relaxed SIstate, where solvent reorganization has already occurred. Electrochromism applied to the absorption spectrum can give the excited-state dipole moment of the Franck-Condon (FC) state and hence the obtained is devoid of the effects due to any solvent reorganization or geometric changes for the molecule. We have used this method to determine the p e ) of ~ various aromatic compounds? including compounds 1-3. The pis obtained for 1-3 were correlated with the degree of twisting of the donor group away from the plane of the ring and its implication to the TICT model? In this paper we describe the ground- and excited-state dipole moments of compounds 4-6,and the results are discussed in terms of the "minimum overlap" rule, proposed by Grabowski.Io 2. Theory and Experimental The theory6 and experimental setup used to obtain the excited-state dipole moment have appeared in our previous publications and here the relevant equations and approximations are briefly summarized. For a dipole molecule, the perturbation due to the presence of an external electric field on the absorption intensity is mainly due to the following factors: (a) anisotropic molecular distribution and (b) change in the electronic transition energy if the ground-state dipole moment is different from pe The field-induced change in molar extinction coefficient (At) is given by eq 1-4.6
(12) Baumann, W.; Petzke, F.; Loosen, K. Z.Naturforch. 1979,34a,1070. (13) (a) Muralidharan, S.; Yates, K. J . Chem. Soc., Chem. Commun. 1991, 250. (b) Muralidharan, S.;Yates, K., manuscript in preparation.
Figure 1. Absorption spectra of compounds 4-6 in dioxane: (a) compound 6 (1.22 X IO-' M); (b) compound 5 (7.75 X IO-s M); (c) compound 4 (1.24 X lo-' M); (d) spectrum c with 0.2 mL of concentrated HZS04.
In the above equations Fin,is the corrected electric field at the location of the molecule in solution; pg is the ground-state dipole moment; A p is the change in dipole moment upon electronic excitation; fi is a unit vector in the direction of the transition moment; h is Planck's constant, and x is the angle between the electric field and the polarization direction of the exciting light. The factor /3 is (kT)-', k being the Boltzmann constant and T the temperature. The externally applied field was converted to an internal field by using the spherical cavity approximation." For each molecule, the electric field signals were collected at three different values of x ( O O , 2 5 O , and SOo) and for several wavelengths (A) in the region of interest. The signals were collected at each wavelength for 5-10 min, depending on the magnitude of the signal, and averaged. The experimentally observed Ae was then least-square fitted with the derivative terms of eq 1 to obtain the coefficients A,, B,, and C,. Since the contribution from the second-derivative term (eq 1) to the electric field signal was found to be very small, this term was neglected in the regression analysis. The pis and pis were calculated from the slope and intercept of least-squares plots of A, and B, versus (3 cos2x - 1). No correction has been made for the effect of the dipole reaction field or for the contribution of molecular polarizability to the electric field signal in the calculation of dipole moment parameters. The lack of the above corrections probably does not introduce serious error in the experimental parameters; detailed investigation on related compounds, such as 4-(9-anthryl)-N,N-dimethylaniline, has shown that polarizability termscan be neglected as compared to the dipole moment terms.12 The syntheses and photophysical studies of compounds 4 4 will be reported e1se~here.l~~ The compounds used for electrochromic studies were >99.5% pure by gas chromatographic analysis. 3. Results The absorption spectra of compounds 4-6 in dioxane are shown in Figure 1. Similar to the case in a c e t ~ n i t r i l ea, ~long-wave~~ length absorption band was seen in dioxane and is assigned to a charge transfer (CT) absorption band, polarized along the short molecular axis (I&,). The electrochromic studies were carried out on the long-wavelength absorption band in the region 450-500 nm, where the locally excited (LE) r,r* absorption band does
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8519
Charge-Transfer Properties in Anthracene Derivatives
W
I
440
~
460
I
480
~
500
I
t
I
l
~
480480500
l
~
l
~
4604805L
(nm)
Figure 2. Electric field spectra of compounds 4-6 in dioxane at y, = Oo: (a) compound 6 (b) compound 5 (c) compound 4. The internal field strength (Pint)= 3.768 X lo' V/cm and T = 298 i 1 K. TABLE I: Ground- and Excited-State Dipole Moments"of Anthracene Derivatives
compd 4 5 6
C I ~
4.6 i 0.4 4.8 i 0.4 5.6 i 0.5
CIeC
10.0 i 1.0 8.9 i 1.0 8.6 i 1.0
AP 5.4 i 0.5 4.1 i 0.5 3.2 i 0.5
All%* 24.8 i 2.0 19.5 i 2.0 17.8 i 1.0
"Ground- and excited-state dipole moments are in debye (D) units. bObtained from electrochromic data. Measured in dioxane. TABLE 11: Torsional Angles (e) and Ground-State Dipole Moments (1,)of Anthracene Derivatives Obtained by the AM1 Method
compd
p'80
e
4 5 6
4.0 4.1 5.3
73 35.5 (92)b 21.5
" C ( ~ ' S reported are for the optimized geometry. bThe calculation indicates that the nitrogen atom of compound 5 has undergone pyramidalization.
not overlap with the C T band. This is evidenced by the fact that, on acidification, the C T absorption is removed and the spectrum shows only small absorbance above 430 nm in dioxane (Figure Id). The electric field spectra for the compounds 4-6 at x = Oo are shown in Figure 2. The discrete points are the values of Ac obtained from the electric field experiment and the solid lines are theoretical curves obtained by a multiple linear regression fit of the experimental data points to eq 1. The excellent fit indicates that eq 1 adequately describes the field-induced changes in the extinction coefficient. Assuming that the dipole moment and transition moment vectors are parallel, which is valid for molecules having C,, symmetry, the ground-state dipole moment can be obtained from eq 2. The p values could not be obtained directly by the dielectric method t u e to the low solubility of 5 and 6 in cyclohexane. The p,'s were obtained from eq 3, by assuming that p, and pe are parallel. The dipole moment parameters for compounds 4-6 obtained from the electric field experiment are summarized in Table I. The p i s for for compounds 4-6 were also calculated by the semiempirical method (AMl)I4 and the results are shown in Table 11. The torsional angle of the donor group was optimized by using the AM1 method and the ground-state dipole moments reported are for the optimized geometry. The close agreement between the pg values obtained experimentally and those predicted by the AM1 calculation indicates that the assumptions made in calculating pg from eq 2 are valid. 4. Discussion
Along with the TICT model, Grabowski introduced the concept of a minimum overlap rule.'J0 In the ground state, donoracceptor groups, connected through an aromatic framework, tend to take up a conformation where the overlap between the orbitals of the aromatic ring and the donor/acceptor groups are maximal. In (14) Dewar, M. S.J.; Zoebisch, E. G.; Healey, E. F.; Steward, J. J. P.J . Am. Chem. SOC.1985, 107, 3902.
I
l
~
l ~ l LU MO
HOMO
,
Figure 3. Molecular orbital diagram of 9-amino- 1 0-cyanoanthracene. The NH2 group is twisted out of the plane of the anthracene ring by 21.5O (optimized by the AM1 method).
the absence of other effects (e.g., steric repulsion), the groups will be planar with the aromatic ring, thus achieving through conjugation between the donor and acceptor moieties. On excitation, it was proposed that the donor and acceptor groups gets uncoupled.'O Charge transfer, following electronic excitation, will then localize the charge on the donor and acceptor moieties.' One consequence of this will be an increased dipole moment for the twisted conformation in the excited state. Our approach to the investigation of this phenomenon was to determine the excited-state dipole moments of benzene derivatives 2 and 3, where the donor group is already twisted in the ground state due to steric hindranceS9It was found that twisting of the donor group away from the plane of the benzene ring favors charge t r a n ~ f e r . ~ By comparison of the torsional angles of the donor group in the anthracene derivatives 4-6,it can be seen that, with increasing torsional angle, the pg decreases (Table 11). On the other hand, the pe value show a reverse trend; i.e., p, increases with increasing torsional angle. The best measure to compare the extent of charge transfer is the difference between the excited- and ground-state dipole moments (Ap). This value increases as the torsional angle of the donor group is increased. A similar observation was made for the benzene derivatives 1-3, where compound 3 had the maximum Ap value of 9.4D.9 One major difference between the benzene derivatives 1-3 and the anthracene derivatives 4-6 is that the latter have a C T state accessible to excitation. According to the TICT model, the benzene derivatives are postulated to acquire a CT state by twisting of the donor group in the excited state,' and hence this state is inaccessible to direct excitation. The electrochromic studies on the C T absorption band of the anthracene derivatives offer a chance to compare the extent of charge transfer, as measured by the magnitude of Ap, between similar anthracene and benzene compounds. The pJ of compound 3, for which the twist angle is estimated to be 80° in the ground state,ls is 4.5 D.9 The anthracene derivative 4, which has a torsional angle of 73', has a comparable p, value of 4.6D. However, the pc values of compounds 39 and 4 are 13.9 and 10.0 D, respectively. Thus, the extent of charge transfer is greater for the benzene derivative, compared to the anthracene compound, though the donor group is twisted to about the same extent in both cases. These observations agree with an earlier conclusion that the anthracene moiety as a donor/acceptor is not efficient in bringing about extensive charge transfer upon electronic excitation.'* Since the pe values are measured for the FC state, these values are not influenced by solvation effects or conformational change that may occur during the lifetime of the excited state. At the present time, the conformational change that might occur for the donor groups on electronic excitation in compounds 4-6 is not well clarified. However, it is safe to assume that for the dimethyl derivative 4 the donor group will remain twisted in the excited state due to the steric repulsion between the peri hydrogens and the CH3 groups. The smaller values of Ap for the anthracene derivatives in comparison to the benzene derivatives9 can be explained in terms of the molecular orbitals (MO) involved in the electronic transition. In the benzene derivatives 1-3 the substituent (NMeJ group is ~
( I 5 ) Rotkiewicz,
~~
~
K.;Rubasazewska, W. J. Lumin. 1982, 27, 221.
J. Phys. Chem. 1991, 95, 8520-8524
8520
directly involved in the electronic transition. In the twisted conformation, the lowest energy transition is from a highest occupied molecular orbital (HOMO), which is purely localized on the donor group NMe2, to the lowest unoccupied molecular orbital (LUMO), localed on the benzonitrile m ~ i e t y . ’ ~Therefore, .~ the HOMO to LUMO transition results in extensive charge transfer. The MO diagram for 9-amino-10-cyanoanthracene is shown in Figure 3. It can be seen that the transition from the HOMO to the LUMO is localized mainly on the anthracene ring. Consequently, there is no significant change in the electron density at the nitrogen atom of the NH2 group upon electronic excitation, thus reducing the absolute value of Ap. The MO diagrams for compounds 4-5 were similar in nature to that for compound 6. Anthracene derivative with only an acceptor group, such as 9-
nitroanthracene, showed a similar behavior for the Ap value.17 Recent reports by Rettig18 and othersI9 have pointed to the validity of the minimum overlap rule in describing the chargetransfer properties of aromatic molecules having donoracceptor groups. The results described in this paper also confirm the observations made earlier for other compounds exhibiting TICT behavior.
Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council (NSERC) of Canada for financial support for this work. Registry No. 4, 133826-03-6; 5, 135614-62-9; 6, 14789-44-7. (17) Sinha, H. K.; Yates, K. J . Chem. Phys. 1390, 93,7085. (18) Rettig, W. Angew. Chem., In?. Ed. Engl. 1986, 25, 971. (19) BonaEiE-Kouteck5, V.; Michl, J. J. Am. Chem. Soc. 1985,107, 1765.
(16) Ertl, P. Collect. Czech. Chem. Commun. 1990, 55, 1891
n = 0-59 and X = OH, 0, 02,
Chemistry of Large Hydrated Anion Clusters X’(H,O),, and 03. 2. Reaction of CH3CN X. Yang: X. Zbang, and A. W . Castleman, Jr.*
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: February 26, 1991; In Final Form: June 5, 1991)
The kinetics and mechanisms of the gas-phase reactions of acetonitrile with large hydrated anion clusters X-(Hz0),,+s9, X = OH, 0, 02,and 03, were studied in a fast flow reactor under thermal conditions. OH-(H20)n.+1 react with CH3CN at near collision rate via proton-transfer and ligand-switching mechanisms; further hydration greatly reduces the reactivity of OH-(H20)n>ldue to the thermodynamic instability of the products compared to the reactants. On the contrary, for all cluster sizes studied, O-(H20), reacts with CH3CN at near the collision rate via a hydrogen transfer from acetonitrile to the anion clusters. A new reaction channel was found for the reaction of 0-with CH3CNto form CHCN-, which can react further with CH’CN to form CHICN and CH2CN-. Only very slow associations were observed for the reactions of 0,and 0; and their hydrates. The possible application of the present experimental results to an understanding of the hydrolysis mechanism of CH3CN in aqueous solution in the presence of OH- and protons as a catalyst is also discussed.
Introduction Most chemical reactions occur in solution, and a vast number of solution reactions are of an ionic nature. However, a molecular-level understanding of solution chemistry is still lacking for many systems due to extreme difficulties in both experiment and theory to handle the solvent effect and the multiparticle interactions present in the condensed phase.’ Indeed, the intrinsic properties of ions or molecules can be more readily obtained in the gas phase since no interference of a bulk solvent exists there; a promising avenue of research employs clusters in order to bridge an understanding of similarities and differences in properties and reactivity between the gas phase and the condensed phase. It has been found that the effect of solvation can alter not only the molecular properties but also the reaction mechanism. For example, the well-known reaction of acetonitrile in acidic (or basic) aqueous solution can proceed via a proton (or OH-) catalyzed hydrolysis reaction mechanism? CH3CN + 2 H 2 0 -% CH’COOH
+ NH3
(1)
However, in the gas phase,’ CH3CN reacts with H 3 0 + by a proton-transfer reaction mechanism: CH’CN
+ H30+
-
(CH3CN)H++ H 2 0
(2)
Interestingly, adding one water onto H30+ as a ligand will change the proton transfer to a ligand-switching mechanism: ‘Present address: Exxon Research and Engineering Company, Clinton Township, Route 22 East, Annandale, NJ 08801-0998.
0022-3654191/2095-8520$02.50/0
-
CH’CN + H30+(H20) (CH3CN)(H20)H++ H 2 0 (3) Since the reactivities of a large number of ion-molecule reactions are now well established in the gas phase? it is a natural step to study the effect of solvation on simple ion-molecule reactions by clustering a variety of different sizes of solvent molecules onto the ions.s It has been recognized that as the number of the solvent molecules gets large, the gas phase large cluster ionmolecule reactions will begin to show some behavior analogous to those operational in solutions.6 In the past year, for the first time large protonated water clusters H+(H20)n=1-603.7.8 and negative cluster ions X-(H20)nIM9 (X = OH, 0, 02,and O3)+I1were produced and studied under well~
~
~
~~~
~
( I ) For example: Moore, J. W.; Pearson, R. G . Kinetics and Mechanism, 3rd ed.;Wiley: New York, 1981. (2) For example: Daniels, F.; Williams, J. W.; Bender, P.; Alberty, R. A.; Cornwell, C. D.; Harriman, J. E. Experimental Physical Chemistry, 7th ed.; McGraw-Hill: New York, 1970; p 139. (3) Yang, X.; Zhang, X.; Castleman, A. W., Jr. Kinetics and Mechanism Studies of Large Protonated Water Clusters H+(H20), n = 1-60, at Thermal Energy. In?. 5. Mass Spectrom. Ion Processes, ih press. (4) Ikezoe. Y.; Matsuoka, S.; Takebe, M.; Viggiano. A. Gas Phase IonMolecule Reaction Rate Constants through 1986; Maruzen Company: Tokyo, 1987. (5) Keesee, R. G.; Castleman, A. W. In Ion and Cluster Ion Spectroscopy and Structure; Elsevier: New York, 1989; pp 275-327. (6) Castleman, A. W., Jr.; Keeset, R. G . Chem. Rev. 1986, 86, 589. Castleman, A. W., Jr.; Keesee, R. G . Acc. Chem. Res. 1986. 19, 413. Castleman, A. W., Jr.; Kcesec, R. G . Science 1988, 241, 36. (7) Yang. X.; Castleman, A. W., Jr. J . Am. Chem. Soc. 1989, I l l , 6845. (8) Yang, X.; Castleman, A. W., Jr. Temperature and Cluster Size Dependence Studies of Reactions of Protonated Water Clusters with Acetonitrile. J . Chem. Phys. 1991, 95, 130.
0 1991 American Chemical Society
