J . Phys. Chem. 1991,95,6835-6842 attributed to the geometric structure, rather than to the electronic structure. In the casc of large cluster ions, n 1 11, there are enough atoms to close the first shell. One atom of the rest may occupy the central position which is shielded from the exterior. In particular, at n L 13 the reactivity decreases remarkably as shown in Figure 3c. The remarkable decrease in reactivity indicates that the first shell is rigidly constructed at n = 13 for the first time. Then, the most probable structure is either fcc or close-packed hcp in which the V atom occupies the central position. When one more V atom is substituted for a Co atom in the Co12V+cluster, the reactivity is found to increase suddenly. This
6835
reactivity change can be also explained by the geometrical structure as mentioned above. Since the second V atom must be located on the surface of the cluster ion, H2 can react with the surface V atom. Further investigation on the IP measurement is in progress in our group to examine the relation between IP and reactivity.
Acknowledgment. We are grateful to Dr. S. Nonose (the University of Tokyo) for his contribution of the first stage of the study and the stimulating discussion. We acknowledge financial support of a Grant-in-Aid for Scientific Research for Priority Area by the Ministry of Education.
Electronic Spectra of PMhalonltriie Isolated in an Argon Matrix Bryce E. Wilhuus~n,**~ Tbomas C. VanCott,ks Janna L. Rose,Lll Andreas Schrimpf,kl Marceli Koralewski,k# and Paul N. Schatz*** Chemistry Department, University of Virginia, Charlottesville, Virginia 22901, and Chemistry Department, University of Canterbury, Christchurch I . New Zealand (Received: March 1, 1991)
The absorption, emission, and magnetic circular dichroism of phthalonitrile (1,2-dicyanobenzene) isolated in an argon matrix are reported between 19000 and 82000 cm-'.These spectra are interpreted in terms of parent transitions involving the benzene ring and cyano substituents. The matrix isolation technique affords well-resolved vibrational structure, which permits the determination of vibrational frequencies for the two lowest lying singlet excited states. Contrary to the conclusions of earlier workers, the spin-consewing transitions are Franck-Condon allowed.
Introdwtioa Phthalonitrile (Pn, 1.2-dicyanobenzene) is a relatively simple derivative of the prototypical aromatic molecule benzene, and it is of interest to understand how the cyano substituents influence the electronic properties of the parent molecule. In this paper we report absorption, emission, and magnetic circular dichroism (MCD) spectra of Pn isolated in argon matrices (Pn/Ar) over the range 19000-82OOO cm-'. There have in fact been few reports of the electronic spectra of Pn. Takei and Kanda' reported the phosphorescence, at 90 K, of Pn dissolved in ethanol and cyclohexane, and the absorption spectra of Pn in the gas phase, in ethanol, and in cyclohexane Over the range 34OOCb38000 cm-I. Barraclough et ala2reported the vapor-phase absorption spectrum over the same range. Much more recently, Toselli et ala3measured the absorption spectra of Pn in various solvents between 32 000 and 36 OOO cm-I. All of these studies concemed only the lowest energy valence transitions of Pn,and were obtained under conditions that yield relatively broad bands. In contrast, we are able to obtain spectra from the visible into the vacuum ultraviolet region, and by using matrix-isolation techniques, we are able to obtain spectra that are very much better resolved. We are thus able to make detailed vibronic assignments of most transitions and can determine the degree of "allowedness'' of the lower singlet excited states. Experimentnl Section Pn was obtained from Eastman Kodak and used without further purification. Matrices were prepared by subliming Pn from a quartz Knudsen cell and codepositing the vapor with a large excess University of Canterbury. 'University of Virginia. I h n t addrsu: Walter R e d Army Institute of Research Department of Rctroviral Research, Suite 200, 13 Taft Court, Rockville, MD 20850. 'Rewnt addrsu: Rayovac Corporation, Madison, WI. h n t addras: Fachkreich Phyiik der Phillip Univeraitat, D-3550 Marburg, Federal Republic of Germany. # Institute of Physics, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland.
0022-3654/91/2095-6835$02.50/0
of argon onto a cryogenically cooled (typically 5-10 K) LiF or c-cut sapphire window. The matrices were not annelaed before data collection. For emission studies, the samples were cooled by using a closed-cycle helium refrigerator (CTI-Cryogenics) operating at 10 K. Spectra were measured by using a SLM spectrometer at a resolution of 0.5 nm. Absorption spectra of the same samples were obtained with a CARY-2145 spectrophotometer at a resolution of 0.05 nm. Samples for the measurement of MCD were prepared in the bore of a superconducting magnet (Oxford Instruments). Sample temperature, magnetic field, and spectral resolution were, respectively, - 5 K, 3.3 T, and 0.4 nm. MCD and absorption spectra below 47000 cm-' were obtained simultaneously by using a spectrometer that has been described previously.4 Spectra in the vacuum ultraviolet region were measured at the Synchrotron Radiation Center of the University of Wisconsin by using the 1-GeV electron storage ring ('Aladdin") and a 1-m AI SeyaNamioka monochromator with a 1200 lines/mm AI grating overcoated with MgF2. All spectra were recorded digitally and analyzed by computer. Calibrations of the absorbance and MCD were achieved by reference to the spectra of a standard solution of d-10-camphorsulfonic acid? Depolarization of light by our matrix samples was determined by comparing the CD spectrum of the standard placed after the sample with that obtained in the absence of the matrix and was found to be negligible.
-
Results
The absorption, emission, and MCD spectra over the range 19OOO-82 000 cm-' are reproduced in Figure 1. The spectra at ( I ) Takei, K.; Kanda, Y. Speclrochim. Acta 1962, 18, 1201-16. (2) Barraclough,C. 0 . ; Bisaett, H.; Pitman, P.; Thistlewaite, P. J. Aust. J . Chem. 1977,30, 753-65. (3) Toselli, N. B.; Anunziota, J. D.; Silber. J. J. Spcrrochlm. Acta 3988, 4 4 4 157-64. (4) Rose, J.; Smith, D.; Williamson, 8.E.; Schatz, P. N.; O'Brien, M. C. M.J . Phys. Chcm. 1986, 90,2608-15. (5) Chen, 0. C.; Yang, J. T. Anal. k t r . 1977, 10, 1195-207.
Q 1991 American Chemical Society
6836 The Journal of Physical Chemistry, Vol. 95, No. 18, 1991
Williamson et al. ' A l ( 'Alg)
5-".rc"L
I
0
- - - - - - - - - -A -
Absorption
0. 5
0
20000
30000
40000
50000
60000
70000
A
-
IJ.
LJ L9ooo
above -47 OOO cm-l were obtained by using synchrotron radiation. The signal-to-noiseratio of the MCD above -57000 cm-l is very poor and those data have been omitted. The emission spectra have not been corrected for instrumental response and the comsponding intensity units (I) are arbitrary. The bands are labeled in accordance with the discussion in the text. 1.4 t I
1.0
470
0. 0
Figure 1. Absorption ( A in optical density units), MCD per tesla (AAIB) and emission spectra (I) of phthalonitrile in an argon matrix. Spectra
-
202
1
BOOOO
E /&1
1.2
0
/
* 382(381u)
20000
21000
22000
23000
I
1
24000
25000
28000
E/cd
Figure 3. 'Al('A1,) 3B2(3Blu) phosphorescence spectrum of phthab nitrile in an argon matrix. The grid at the top of the diagram illustrates our analysis of the vibrational structure (see text), where the principal progression-building mode (1570 cm-I) is emphasized. The spectrum is not corrected for instrumental response and the intensity (I) is in arbitrary units. +
TABLE I: Electronic Tnnsitiolrr of Phthrloaitrile in an Argon Matrix excited state# p o h b 8 ( 0 8 )/cm-l do/do( I ( I ~ 'BA'BiJ Y 25 7a7c 'Ai('B2U)
Z Y
34 680 41 450
2 ~ ) )
1.o
10.7
0.8
A
I
I
r'
I
34500
I
I
34600
E/"'
34700
Figure 2. (0,O) absorption bands of lAl(IAl,) IAl(lBh) transition of phthalonitrile in an argon matrix. At least six bands (designated by vertical bars) are observed, which are attributed to inequivalent sites in the matrix.
lower energies have been normalized to the same scale as those a t vacuum ultraviolet energies. The lower energy emission band of Pn/Ar (1 9 000-26 000 cm-I, designated P in Figure 1) is similar in appearance to the phosphorescence reported by earlier workers for Pn in ethanol and cyclohexane,' but is better resolved. The most-studied transition of Pn is the lowest energy absorption band'-' covering the region 3400040000cm-'(band A of Figure 1). The Pn/Ar spectrum is far better resolved than earlier solution spectra.'" The finer details of the band envelope centered near 34 680 cm-' are shown in Figure 2. A similar pattern is observed for other envelopes and is attributed to the presence of inequivalent sites in the matrix. This is a well known phenomenon, and in the case of ZnPc/Ar? the presence of at least seven distinct sites was clearly apparent in the sharp spectra observed in the Q-band region. Transitions to the blue of 40000cm-I have not been previously reported. In the region 40oowO000 cm-l there are two intense electronic transitions, (designated B and C in Figure 1) both of which exhibit considerable vibrational structure. Between 60000 and 80000 cm-I a broad absorption envelope is observed, with a maximum at -69000 cm-I (E)and a weaker shoulder centered a t -63000 cm-I (D). ( 6 ) VanCott, T. C.; Rose, J. L.; Miscner, G. C.; Williamson, B. E.; Schrimpf, A. E.;Boyle, M.E.;Schatz, P. N. J. Phys. Chem. 1989, 93, 2999-301 1.
OState symbols in parenthesis indicate the parentage of the excited state (see text). bBothy and z lie in the molecular plane with I collinear with the 2-fold symmetry axis. CObtainedfrom the phosphorescence spectrum. dRough estimates from the absorption s rum. It is not possible to determine the relative energies of I A l ( G ) and IB2(CIA'). The MCD spectrum is shown at the top of Figure 1. The signal-to-noise ratio (S/N) above 57 000 cm-' is very low, due to the weak MCD in this region, and the corresponding data have been omitted. The MCD of bands A and B is single-signed (respectively positive and negative) and is similar in appearance (apart from a lower S/N) to the absorption. This is indicative of MCD 2l terms,' which arise from field-induced mixing of states. They normally only dominate when the transition occurs between nondegenerate states.' The MCD of the vacuum ultraviolet bands near 50000 cm-' (C) shows both positive and negative features. This type of MCD dispersion is often an indication of degeneracy in the gound and/or excited state and is known as an A term.' However, the fact that the maxima and minima of the MCD are coincident with maxima of the absorption shows that the MCD actually arises from overlapping 9 t e r m ~ . ~The J overall derivative-like dispersion is known as a p s e u d o 4 term6j and is commonly associated with a lowering of symmetry, which splits the energies of formally degenerate states.6 Discussion Pn is a planar molecule with C, symmetry and a 'Al electronic ground state. Following VarsBnyi* we define the molecular reference frame so that the z direction coincides with the 2-fold symmetry axis and the x direction is perpendicular to the molecular plane. The allowed electric-dipole transitions from the ground state are 'Al 'A,, IBI, IB2. (We follow the convention7that the lower
-
(7) Piepho, S.B.; Schatz, P.N. Group Theory in Spectroscopy wiih A p plicaiiow io Magnriic Circular Dichroism; Wiley: New York, 1983. (8) Varshyi, G. Vibraiional Spectra of Benzene Dwioatiurs;Academic
Press: New York, 1969.
The Journal of Physical Chemistry, Vol. 95, NO.18, 1991 6831
Spectra of Phthalonitrile Isolated in an Argon Matrix
--
energy state is always to the left, so that absorption and emission are distinguished by the direction of the arrow.) 'Al lAl, IB2 are polarized in the plane of the molecule while 'Al 'B, is polarized perpendicular to the plane. As a first approximation, the transitions that we observe for Pn can be correlated with the transitions of the benzene and cyano chromophores. In the following, this formal parentage is denoted in parentheses. For example, the phosphorescence (Figure 3) is designated 'AI('Ala) 3B2(3Blu)and is correlated with the 'Alg 3Bluphosphorescence of benzene. The assignments are summarized in Table I. In accordance with earlier workers,'-3 we correlate the transitions below 6OOOO cm-I with the valence transitions of benzene. The assignments at higher energy are less certain. They are too intense to be associated with the 2R Rydberg transition of benzene in argon,9and it seems more likely that they are associated with the X(lZ+) &A') transition of HCN.'O*'' Assuming this assignment and that Pn retains C, symmetry, there should be two such excited states, with 'Al and 'B2 symmetry (Table I). All of the spin-conserving transitions we observe are formally allowed in C, symmetry. However, below -47000 cm-I the (benzene) parent transitions are formally forbidden and gain intensit through vibronic coupling with the 'Elu state via e g modes.'l In general we might therefore expect the total intensity of a transition of Pn to comprise contributions from "allowed" and vibronic mechanisms, both of which can contribute to the vibrational structure. We can distinguish vibrational side bands that arise from each of these mechanisms by comparing the MCD with the absorption. For a transition lAI('Ala) J of Pn, the absorbance (A) and MCD (PA) are given by7 A / & = (AL + AR)/26 = 326.6dao(J)f(C) (1) P A / & ( A L- A R ) / & 152.5E~lBo(J)f(C) (2) AL and AR are, respectively, the absorbance of right and left circularly polarized light. C = hu is the photon energy (cm-I), c is the sample concentration (mol dm"), 1 is the sample path length (cm), E is the magnetic induction (T) andf(C) is a nordC = 1. Do(J) is the dipole malized band-shape function; strength of the transition 'AI('Al8) J and includes contributions from both "allowed" and vibronic mechanisms. Bo(J) is the corresponding Faraday B term.' (The Faraday terms A,(J) and e0(J) vanish since Pn has no orbitally degenerate states.) These parameters can be extracted from the experimental data by numerical integration
-
-
-
-
-
sf(&)
B0(J) = l ( M / C ) dC/152SEcl
(3)
S0(J) = I ( A / C ) dC/326.6cl
(4)
If we assume random orientation of the guest molecules then the theoretical expressioy for the corresponding space-averaged parameters, bo(J) and B0(J) are7 bo(J) = 731('AI ('A1g)lmlJ) I2 (5)
In q 6, Im indicates imaginary part, W(K) and W(J) are electronicenergies, m is the electric-dipole operator, and L is the orbital angular momentum operator. We have assumed that the (9) Boyle, M. E.; Williamson, B. E.; Schatz, P. N.; Marks, J. P.; Snyder, P. A. C h w . Phys. Len. 1986, 130, 33-8. (10) Herzbcrg, G. Elecrronlc Specrra and Electronic Srrucrure of Polyaromfc Molecules; Van Nortrand New York, 1966. Herzberg, Go;lnna,
K.K. Can. J . Phys. 1957,35, 842-19.
( 1 1 ) Schwcnzer, G. M.; ONeil, S. V.; Schaefer, H. F.; Barkin, C. P.; Bender, C. F. J . Chem. Phys. 1974,60, 2181-93. (12) Ziegler, L. D.; Hudson, B. S. In Excited Srarcs; Lim, E. C., Ed.; Academic Press: New York, 1982; Vol. 5 (and reference therein).
ground state is sufficiently well separated from any excited states thal field-induced mixing only between excited states contributes to Bo(J). By use of standard first-order Herzberg-Teller theory, the electric-dipole matrix element common to eq 5 and 6 is ('Ad'Ala)linlJ) = ('Al('Ala)14J)o +
The zero superscripts on the right side of eq 7 mean that the corresponding matrix elements pertain to unperturbed electronic states in their equilibrium nuclear configuration, and we have again assumed mixing only between excited states. The normal coordinates of the molecule are represented by Q, and the corresponding vibrational quantum numbers for the ground and excited state are designated u, and u,', respectively. Overlap factors involving the vibrations in the intermediate electronic state, K, do not appear due to closure of the sum over a complete set. The operator UQ,is given by eq 8 where V is the potential energy.
ua ( ~ V / ~ Q O Q , = O
(8)
Now consider an individual vibrational side band corresponding to the excitation of u,' quanta of Q, in the final state. The temperature is sufficiently low so that u, = 0 in the 'Al('Ala) ground state, and the transition moment is ('A1(lAlg),olmlJ,u,') = ('A1('A1g)lmlJ)o(Ol~,')+ ('Al(lAlg)lmlJ)'~(OIQrlu,') (9) ( 'AI('A1#n(J)'a is an effective matrix element, which represents the sum in square brackets of eq 7. If we assume that the molecular symmetry is unchanged during the transition, then the vibrational overlap factors (Olu,') will be nonzero only if u; = 0 or if the mode Q, transforms as the totally symmetric irreducible representation, al. (We denote the symmetry of vibrational modes in lower case to distinguish them from electronic states.) Thus, the first term of eq 9 gives rise to a zero-phonon origin (0,O)upon which can be built Franck-Condon progressions in totally symmetric vibrational modes. We describe the vibrational side bands corresponding to this term as arising via a Franck-Condon (FC) mechanism. By use of the harmonic approximation, the only nonvanishing contribution to the second term of eq 9 contains the integral (OlQ,lu,' = 1). This gives rise to a false, Herzberg-Teller origin shifted by one quantum of a non-totally-symmetric mode Q,from the (0,O)band. We describe such false origins (and the a1 progressions that can be built on them) as arising via a HerzbergTeller (HT) mechanism. For a single line arising from the FC mechanism, we assume that vibrational energies are small compared with energy differences between electronic states. Then
[ b o ( J ~ , )Fc l =
%I
( 'AI (' AiJlmlJ
)OI21 (Olv'l, )I2
(1 0)
Williamson et al.
6838 The Journal of Physical Chemistry, Vol. 95, No. 18, 1991 With the approximations above, the 9 terms due to the HT mechanism are zero, since the overlap factor (u', = 110) vanishes for non-totally-symmetricmodes. Even if we allow_somerelaxation of our approximations, we anticipate the ratio Bo/b0for a line arising from the HT mechanism to be very much smaller than (and possibly of the opposite signi3," to) that of the electronic origin. If both mechanisms contribute significantly to the overall transition intensity, it should be possible to determine which mechanism is responsible for each vibrational side band on-this basis. In particular, any band for which the value of Bo/B0is very much less than that for the (0,O)band must arise from the HT mechanism. There is reason to believe that planar molecules in noblegas matrices can take a preferential orientation with their molecular planes parallel to the surface of the matrixdepositionwindow.6J5J6 The degree of orientation appears to be dependent on the conditions of deposition. For example, ZnPc/Ar matrices deposited on a LiF window at a nominal temperature of - 5 K exhibited strong orientational effects? whereas ZnPc/Ar obtained under the same conditions but with a sapphire window showed a more random orientation.l6 If the Pn molecules in Pn/Ar exhibit strong orientational effects of this type, then for the case where the radiation propagates perpendicular to the maxtrix window and excited states are restricted to those of AI and B2orbital symmetry, we would expect to obtain6 BOX(J)= 3/2330(J)
(14)
Box(J) = 3&(J)
(15)
Box(J)/W(J) 2%J) /Bo(J)
(16)
and hence
The x superscriptson the left side indicate that these parameters pertain to the case where the optical axis is parallel to the molecular x axis. All of the transitions that we observe are believed to involve A, or B2 excited states (seeabove). Hence we expect the Bo/ll0 ratio to vary if different preparation conditions produce different degrees of orientation, and indeed we have observed variations over a factor of about 2. The lowest value of Bo/B0is obtained from Pn in a polyvinyl alcohol film" in which we can be certain that the guest molecules are randomly oriented. This is compelling evidence that Pn can take a preferential orientation under suitable conditions. The data presented in this work were obtained from matrices exhibiting low Bo/B0ratios suggesting that the Pn molecule orientations were largely random. In the following we assume c_ompletely random orientation and use the parameters boand BBo.The actual degree of preferential orientation does not in any way negate our arguments concerning the relative importance of the FC and HT mechanisms because the proportionality in eq 16 applies to both mechanisms. We note in passing that we reported the MCD and absorption of benzene in an argon matrix (C6H6/Ar)9before we were aware of the propensity of planar molecules to show preferential orientation in noble-gas matrices. The ratio of the MCD to the absorbance for the !Al, lElutransition ( A i / D o in that case) was found to be nearly 3 times greater for C6H6/Ar than for benzene in solution.18 At the time we ascribed this observation to a change in the magnetic interaction of the lEluvalence state with the higher lying 2R Rydberg state as the environment of the
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(13) Barton,T.J.; Douglar, 1. N.; Grinter, R.; Thonuon, A. J. Mol. Phys. 1915, 30, 1677-84. Grinter, R. In Matrix fsolutlonSpectroscopy; Barnes, A. J., Orvillc-Thomah W. J., M(Ulcr, A,, Gaufrb, R., LIS.; Reidel: Dordrecht.
The Netherlands, 1980, pp 49-90. (14) Rmfield, J. S.;MoKmwitz, A,; Linder, R. E. J . Chem. Phys. 1974, 61, 2427-37. (IS) Swanson, B. 1. Private communication. (16) Metcalf, D. H.; VanCott, T. C.; Snyder, S.W.; Schatz, P. N.; Williamson, B. E. J . Phys. Chrm. 1990, 94, 2828-32. (17) Jack, K. S.;Williamson, B. E. Unpublished results. (18) Fuke, K.; Gedanken, A,; Schnepp, 0. Chrm. Phys. fen. 1979,67, 483-6.
benzene molecule is changed? We now believe that the effect is due, at least in part, to preferential orientation of the guest molecules in the argon matrix. All of the transitions in Figure 1 involve Ai or B2excited states, which can be coupled via a l and b2 vibrational modes. We therefore expect to observe false origins involving b2 modes and progressions of ai modes. In particular, in light of the predominant HT mechanisms for the forbidden transitions of benzene (coupling to admixed )Eluvia e, modes12),we expect to observe ai and bz vibrations that are refated to ea modes in Dah symmetry. In the following, we discuss the vibrational structure associated with the individual electronic transitions. In doing so, we make use of the vibrational data and assignments of Barraclough2and Castro-Pedrozo and KingIgfor the ground state. These earlier workers classified the normal modes of Pn, using the Wilson notation for benzene,Mand the methods described by VarsBni? Although such formal classifications may be qualitatively useful, they are often arbitrary and can be misleading. For example the in-plane C-H deformation modes (4 H) of benzene are denoted v3(aa), v9(ezB),vi&,,) and vdei,,).S For Pn, the PC-CN modes 5 of ai and b2 symmetry have been variously classified as ~ 1 and u18,,Z ug, and ~ ~ and~ ~ 1 ~and 5 , vgki9 8 The fact that the &
