J. Phys. Chem. 1983,87,280-289
280
previously demonstrated in this laboratory. However, the concentrations used were much higher than in clinical practice and for this and other obvious reasons the results presented in this paper do not involve any connection between anesthetic action and the dissociation of H bonds in nucleic base pairs. The intensity changes of the free and associated infrared NH stretching bands and H-bond changes in the NMR chemical shift show conclusively that the H-bond equilibrium in the base pair is strongly perturbed by the anesthetics, many of the H bonds being dissociated. As to the potent general anesthetics, chloroform, halothane, methoxyflurane, enflurance, the perturbation goes mainly through the establishment of an equilibrium of the type2f~23~24 N-H**.:O=C or N-H**.:N-C
+ H-CCl, * C=O:***H-CCl, + NH + H-CC13 * C=N:*..H-CC13 + NH
(23) P. Hobza, F. Mulder, and C. Sandorfy, J . Am. Chem. SOC., 103, 1360 (19811. (24) P. Hobza, F. Mulder, and C. Sandorfy, J. Am. Chem. Soc., 104, 925 (1982).
For phenobarbital our results clearly show that the EA/CU complex is destroyed in favor of an EA/PB complex, which implies the dissociation of the H bonds in the former and the formation of new ones in the latter. This work is meant to be the beginning of thorough investigations on the H bonds in the isolated Watson-Crick base pairs, as well as in a molecular environment approximating in vivo conditions. It is hoped that a scheme will be found making it possible to selectively interfere with these vital H bonds. This could open new perspectives. Acknowledgment. We are indebted to Professor Jacques Weber from the University of Geneva, Switzerland, for making our collaboration possible. We thank Dr. R. Denis for stimulating discussions on anesthetic molecules and Mr. Robert Mayer for assistance in measuring the NMR spectra. Financial assistance from the Natural Sciences and Engineering Research Council of Canada and the Minist6re de 1'Education du Quebec are gratefully acknowledged as well as an I. W. Killam memorial scholarship by the Canada Council to C.S. Registry NO.CU, 712-43-6;EA, 2715-68-6; PB, 50-06-6; CHC13, 67-66-3; CHFC1CF20CF2Hl 13838-16-9;CHzC12, 75-09-2; CCld, 56-23-5; CF3CHC1Br, 151-67-7.
Lowest Excited Singlet State of Hydrogen-Bonded Methyl Salicylate LouAnn Helmbrook, Jonathan E. Kenny,+ Bryan E. Kohler,' and Gary W. Scott* Department of chemistry, Weskyan Unlversity, MMietown, Connecticut 06457 (Received: August 2, 1982; I n Final Form: September 23, 1982)
The dual fluorescence shown by methyl salicylate in solution and the room temperature vapor phase has been observed for methyl salicylate seeded in a supersonic helium jet. Under these conditions low rotational and vibrational temperatures are attained, making it possible to measure highly resolved excitation spectra for each of the emission components. This paper reports the measurement of these excitation spectra together with emission spectra and emission decay kinetics as a function of excitation wavelength for both methyl salicylate-h and methyl salicylated in a supersonic jet. The measured spectra are analyzed to determine the nature of the excited states responsible for the dual emission.
1. Introduction
It has been known for many years that the internally hydrogen-bonded methyl salicylate molecule displays dual fluorescence in solution as well as in low-pressure gasses.'-3 Following established usage, these different fluorescence spectra are designated blue (maximum emission intensity a t 440 nm) and UV (maximum emission intensity a t 335 nm). Recently we reported that this dual emission could also be observed under the low-temperature isolated molecule conditions that obtain in a free jet.4 This is in contrast to the situation in the low-temperature condensed phase where only the blue emission is observed: indicating that the two forms of the molecule responsible for the two different emission spectra do not equilibrate during the expansion. This has allowed us to obtain well-resolved excitation spectra for both hydrogen-bonded species. These spectra together with studies of the fluorescence Present address: Department of Chemistry, Tufts University, Medford, MA 02155. Permanent address: Department of Chemistry, University of California a t Riverside, Riverside, CA 92521.
*
0022-3654/83/2087-0280$0 1.50/0
decay kinetics as a function of excitation and detection conditions afford further insight into the electronic states and potential surfaces for both hydrogen-bonded species. The measured excitation spectra, first reported in our preliminary communication,4together with the kinetic data are presented in detail, analyzed, and interpreted in this paper. Although the dual fluorescence of methyl salicylate was initially thought to originate from an excited-state equilibrium between tautomeric forms of the molecule which are related by proton t r a n ~ f e r ,there ~ , ~ is now conclusive evidence that it arises from an equilibrium in the ground state between two different forms of the molecule that do not equilibrate on the excited-state ~ u r f a c e . ~ - Studies '~ (1) J. K. Marsh, J. Chem. SOC.,126, 418 (1924). (2) A. Weller, Z. Elecktrochem., 60, 1144 (1956). (3) A. Weller, Prog. React. Kinet.,1, 188 (1961). (4) L. A. Heimbrook, J. E. Kenny, B. E. Kohler, and G. W. Scott, J. Chem. Phys., 75, 5201 (1981). (5) J. Goodman and L. E. Brus, J. Am. Chem. Soc., 100, 7422 (1978). (6) K. Sandros, Acta Chem. Scand., Ser. A, 30, 761 (1976). (7) K. K. Smith and K. J. Kaufmann, J.Phys. Chem., 82,2286 (1978). (8) W. Klopffer and G. Kaufmann, J . Lumin., 20, 283 (1979).
0 1983 American Chemical Society
Singlet State of Methyl Salicylate
The Journal of Physical Chemkrry, Vol. 87, No. 2, 1983 281
of methyl salicylate in the gas phase,&ll including our free jet ~ t u d i e sare , ~ consistent with the assignment of these two ground state forms to different rotamen (I and 11)with OCH,
I
0
II
'B
7
3
ONH
I
I1
different intramolecular hydrogen bond partners. The more abundant form I gives rise to the blue fluorescence (maximum intensity at 440 nm) and the UV fluorescence (maximum intensity at 335 nm) is associated with the less abundant species 11. In the room temperature vapor we estimate that I1 is present at approximately 1/70 the concentration of I (see section 2.2.3.). The fluorescence of form I exhibits several interesting features. There is an unusually large difference in energy between the maxima in the absorption and fluorescence spectra (Stokes shift). The quantum yieldlo and decay kinetics" of the blue emission of vapor-phase methyl salicylate show a dramatic dependence on excitation energy. In solution the lifetime of this emission is a strong function of the properties of the solvent, particularly the solvent dielectric constant.18 The large Stokes shift between the absorption and emission maxima for conformer I has been attributed to a large difference between the ground- and excited-state geometries involving the shift of the proton in the hydrogen bond toward the carbonyl oxygen (tautomerizati0n).~8In solution the blue emission has a risetime of less than 10 ps' which, in this model, seta a lower bound on the rate of proton transfer in the excited state. Approximate molecular orbital calculations on the related salicylic acid molecule indicate either a small barrier to proton transor no barrier at all.17 Spectroscopic studies of methyl salicylate in the low-temperature condensed phase do not provide any evidence for a barrier to proton transfer in the excited state of form L5J7Higher resolution spectroscopic studies on methyl salicylate under cold isolated molecule conditions can, in principle, give sufficient information to permit a determination of the height of the barrier to proton transfer in the excited state of species I. In the jet the excitation spectra are in fact resolved enough to support a detailed vibrational analysis. However, for species I we find that the onset of an efficient radiationless channel for excitation energies that deposit more than 1000 cm-l in the excited state eliminates the possibility of seeing modes associated with the -0-H-Ostretch coordinate. While the spectra do not extend to high enough energy so as to unambiguously determine the extent of proton transfer following excitation, they do place constraints on the excited-state potential. In this paper (9)A. Acuna, F. hat-Guerri, J. Catalan, and F. Gonzalez-Tablas, J. Phys. Chem., 84,629 (1980). (10)A. Acuna, J. Catalan, and F. Toribio, J. Phys. Chem., 86, 241 (1981). (11)R.Lopez-Delgado and S. Lazare, J. Phys. Chem., 86,763(1981). (12)D. Ford, P.J. Thistlethwaite, and G. J. Woolfe, Chem. Phys. Lett., 69,246 (1980). (13)J. Catalan, F. Toribio, and A. U. Acuna, J.Phys. Chem., 86,303 (1982). (14)J. Catalan and J. I. Fernandez-Alonso, J. Mol. Struct., 27, 59 (1975). (15)J. Catalan and F. Tomas, Adu. Mol. Relaxation Processes, 8,87 (1976). (16)G.W. Scott and D. Vosooghi, unpublished results. (17)R. D.h u h , Ph.D. Thesis, Princeton University, 1972. (18)K. K. Smith and K. J. Kaufmann, J. Phys. Chem., 86, 2895 (1981).
we discuss these constraints. Finally, as reported previ~usly,~ the UV emitting form I1 of methyl salicylate is also present in our free jet. Again, the excitation spectra are well resolved and provide information on the nature of the excited state of the UV emitting form. Additionally, by measuring the relative intensities of the blue and UV fluorescence spectra as a function of reservoir temperature, we have been able to estimate the ground-state energy difference between forms I and 11. 2. Experimental Section 2.1. Samples and Techniques. Methyl salicylate (Aldrich), hereafter referred to as methyl salicylate-h, was purified by vacuum distillation and the purity was checked by gas chromatography. Methyl salicylate in which deuterium replaces the phenolic hydrogen, hereafter referred to as methyl salicylate-d, was prepared by adding a 20-fold molar excess of deuterated methanol to a methyl salicylate-h samples and allowing the mixture to stand for several hours. This deuterated sample was purified by fractional distillation and stored under nitrogen until used. The sample obtained by this procedure consisted of approximately 50% methyl salicylate-h and 50% methyl salicylate-d as determined by NMR and mass spectrosCOPY. The methyl salicylate samples used to seed the free jet consisted of 2 mL of liquid contained in a stainless steel chamber which could be heated to approximately 150 "C. Helium carrier gas at a controlled pressure from 1to 15 atm flowed through this chamber just prior to expansion into an evacuated (0.1-0.2 torr) chamber through either a 50-, loo-, or 200-pm diameter nozzle. At room temperature, the vapor pressure of methyl salicylate in the preexpansion mixture is 0.2 torrs giving a mole fraction of methyl salicylate in the reservoir (and presumably in the seeded jet) which ranged from 20 to 300 ppm. The free jet spectrometer and techniques used have, in part, been described elsewhere.20 At a distance of 1-5 mm from the nozzle the methyl salicylate seeded jet was intersected at a right angle to the direction of flow by the frequency-doubledoutput of a Molectron DL-400 dye laser pumped by a 1MW Molectron UV-24 nitrogen laser. The dyes used to measure excitation spectra from 3410 to 2790 A were all obtained from Exciton and included DCM in dimethyl sulfoxide, and rhodamine 610, rhodamine 590, and coumarin 540A in ethanol. The second harmonic pulses typically contained 1-20 pJ each, were approximately 6-1-15wide, and had a spectral bandwidth of 1to 1.5 cm-'. These pulses were focussed by a 1.25-m focal length spherical mirror to a beam waist at the jet axis of roughly elliptical cross section 100 X 300 pm. So that the emission spectra described below could be obtained, the emission from the jet-cooled methyl salicylate molecules was spectrally dispersed through a 0.3-m monochromator (McPherson Model 270 typically with 1.9-mm slits to give a resolution of approximately 40 A) before being detected by a cooled photomultiplier (EM1 6256B). Emission spectra were obtained by integrating the photocurrent produced by 200 laser pulses at each wavelength. Points were taken every 5 A. In these experiments the nozzle diameter was 200 pm; the helium pressure in the reservoir was 1.5 atm and the laser beam waist crossed the jet axis 1.3 mm downstream from the (19)E. Rajogopal, K. V. Sivakumar, and S. V. Subrahmanyam, J. Chem. Soc., Faraday Trans. 1, 77,2149 (1981). (20)L. A. Heimbrook, J. E. Kenny, B. E. Kohler, and G . W. Scott, J. Chem. Phys., 76,4338 (1981).
282
Heimbrook et al.
The Journal of Physical Chemlstty, Vol. 87, No. 2, 1983
r a
x
i
-
30030
1
30330
30630
1 1
30930
FREQUENCY ( c m - l
' +A -
I' 31230
)
+
01 C
ar
.p
K H
30030
30330
30630
30930
FREQUENCY ( c m - l
31230
31530
)
Figure 1. (a) Excitation spectrum of the blue emission of methyl salicylate-h in a free jet. Only the low-energy region within 1500 cm-' of the origin is displayed. Additional details are g h m in the text. (b) Excitation spectrum of the blue emission of an isotopically mixed sample of methyl salicylate-d and methyl salicylate-h in a free jet.
orifice. Emission intensities were not corrected for the wavelength response of the detection system. In the excitation mode the emission was viewed with the cooled photomultiplier through an appropriate system of lenses filters and masks perpendicular to both the jet axis and the line of excited molecules. Either 100,200, or 400 pulses (at a 23-Hz repetition rate) were used to generate each point in the excitation spectra presented. To obtain excitation spectra of the blue emitting species (I), we replaced the monochromator with a filter that transmitted all light to the red of either 370 (Schott KV 370) or 389 nm (Schott KV 389). To obtain the excitation spectrum of the UV emitting species (II), we used a different filter combination consisting of 70 g/L of an aqueous solution of NiS04 in an 0.8-cm pyrex cell, a UV-transmitting blue-blocking filter (Schott UG5,2 mm thick), and a suitable UV cutoff filter to block scattered laser light (either a Schott WG 345, 1 mm thick; a WG 320, 3 mm thick; a WG 320, 1mm thick; or a WG 305, 1 mm thick, depending on the excitation wavelength.) Points in the excitation spectra were taken every 0.1 %, (1 em-'). The nanosecond fluorescence kinetics of both the methyl salicylate-h and methyl salicylate-d were obtained with the free jet apparatus in the excitation mode as described above except that the photomultiplier was replaced with one exhibiting faster time response (Amperex XP-1002) whose output was monitored with a Tektronix 7912AD programmable digitizer. Data collection and analysis utilized a Hewlett-Packard 9835A computer. Each decay curve was least-squares fit by the convolution of the system response (determined in a separate experiment by measuring the time profile of the laser light scattered off a pinpoint placed at the laser-jet intersection) with an exponential decay. 2.2 Results 2.2.1. The Blue Emission of Methyl Salicylate. 2.2.1.1. Excitation Spectra. The excitation spectrum of the blue emission of methyl salicylate-h seeded in a freely ex-
panding helium jet was shown in Figure 1 of our earlier That spectrum showed a sequence of sharp vibronic lines, including a progression in a 176-cm-' low-frequency mode, built on an origin at 30052.3 cm-'. The spectrum becomes quite congested 700 cm-l above the origin and becomes virtually a continuum 1500 cm-' above the origin. The sharp low-energy features of this spectrum are shown in Figure la. A complete listing of the observed bands and a set of vibrational assignments are given in Table I. The excitation spectrum of the blue emission of the mixture containing approximately50% methyl salicylate-d with 50% methyl salicylate-h was also determined in the same energy region (Figure lb). A number of new bands appear in the spectrum of the mixture which we attribute to methyl salicylate-d. The energy positions of the intensity maxima of these new bands are listed on the right side of the Table I in the same row as the analogously assigned bands of the methyl salicylate-h molecule. The electronic origin band of the methyl salicylate-d blue excitation spectrum occurs at 30 151.1 cm-l-a shift of 99 cm-' to higher energy with respect to methyl salicylate-h. Although the mixture contains approximately equal amounts of methyl salicylate-h and methyl salicylate-d, the intensity of the origin peak is significantly weaker for the deuterated compound. Additional experiments were done to establish that the line at 30052.3 cm-l was the origin of the blue-emission excitation spectrum. Figure 2 shows the line shape of the origin band as a function of preexpansion helium pressure. As the pressure is increased, a new feature appears 5 em-' to higher energy from the origin. This peak is assigned to a van der Waals complex between methyl salicylate and helium. A t the lower reservoir pressures used for determining the spectra of Figures 1,3,5,and 6 there were no peaks assignable to complex species. Relative intensities of the excitation features were also not changed by the addition of water to the sample. 2.2.1.2. Fluorescence Spectra. Blue emission spectra of methyl salicylate-h obtained at two different excitation
The Journal of Physical Chemistry, Vol. 87, No. 2, 1983 283
Singlet State of Methyl Salicylate
r
r t
i
_
L
u
_
i
l
~
l
l
n
~
36045
30050 30055 30060 30065 'REQUENCY (cm-l 1 F@m 2. Effect of the preexpansion hellum pregwre on the line shape of the origin band In the excitation spectrum of the blue emlsslon of methyl salicylate-h. The preexpanslon pressures from the top to bottom spectrum shown in this flgue are 88,4428, and 14 atm. Each band has been normalized to the same maxlmum lntenslty value, and the baselines have been displaced for clarity.
0
40
80
Time
120
160
200
(nsec)
Figure 4. Typical blue fluorescence decay kinetics of methyl sallcylatah obtained by exciting at the orlgln of the lowest excited singlet state. The squares (0)give the experimental points, and the smooth curve Is a least-squares best fit obtained by deconvolutlon from the Instrument response to the laser excitation pulse, assuming single exponential decay. The Itfetlme deduced from these kinetics was 1 1.3
i 0.1 ns.
18500
21000
23500
26000
28500
FREQUENCY ( c m - l 1
'V FREQUENCY ( c m - l 1 Figure 3. (a) Blue emlsslon spectrum of methyl salicylate4 In a free jet excited at the origin (30 052.3 cm-') of the lowest excited slnglet state. This spectrum was obtalned at 40-A resolution and was not corrected for detector response. (b) Blue emission spectrum of methyl salicylate-h In a free jet, excited at +lo94 cm-' ( Y , of ~ Table I) above the origin of the bwest excited singlet state. Condltlons were othemise the same as in a.
energies-(a) 30052.3 cm-' (origin) and (b) 31 146.4 cm-' (v13 = 1094 cm-')-are shown in Figure 3. The FranckCondon maximum of the origin excited emission occurs at 437 nm (Figure 3a) while that of the ~ 1 vibronic 3 band excited emission occurs at 445 nm (Figure 3b). The maximum in the emission spectrum shifts to higher energy as the excitation energy is increased above the 0-0. 2.2.1.3. Fluorescence Lifetimes. Fluorescence decay times for the blue emission were determined for excitation of distinct vibronic bands in the low-energy excitation region for both methyl salicylate-h and methyl salicylated. The lifetimes were extracted by least-squares fitting the observed decay curves by the convolution of an exponential decay with the measured laser pulse profiie. The observed decay curves were well fit by a single exponential. A
typical decay curve together with the results of the least-squares analysis is shown in Figure 4. The lifetimes are summarized in Table I1 (see Table I for the numbering of the modes.) 2.2.2. The UV Emission of Methyl Salicylate. 2.2.2.1. Excitation Spectra. The excitation spectrum of the UV emission of methyl salicylate-h seeded in a freely expanding helium jet was shown in Figure 2 of our earlier r e p ~ r t .This ~ spectrum is reproduced here in Figure 5a. A listing of the energies of the sharp bands observed in this excitation spectrum is given in Table I11 together with some vibrational assignments. The excitation spectrum of the UV emission of the sample containing methyl salicylate-d was also obtained, and it is shown in Figure 5b. The energy positions of the intensity maxima of the new bands that are observed for this sample are listed on the right side of Table 111, in the same row as the analogous bands of the methyl salicyalte-h molecule. The 0-0bands of methyl salicylate-h and methyl salicyalte-d in the deuterated sample occur with roughly equal intensities in accordance with the composition of the sample. The origin band of the methyl salicylate-d UV excitation spectrum is at 21 cm-l higher energy than the origin band of the methyl salicylate-h UV excitation spectrum. 2.2.2.2. Fluorescence Spectrum. The low-resolution ultraviolet (and blue) emission spectrum of methyl salicylate-h excited at 33 131 cm-I (Y, of Table 111) is shown in Figure 6. This excitation energy overlaps both a sharp vibronic band in the W-excitation spectrum as well as the continuous absorption tail of the blue excitation spectrum. The maximum of the W emission occurs at 3350 8,in this spectrum. An attempt was made to obtain the fluorescence lifetime of the UV emission. The lifetimes are sufficiently short compared to the excitation pulse width (6 ns) that we were unable to obtain an accurate value. The fluorescence lifetime of the UV emitting species must be less than 2 ns. 2.2.3. Temperature Dependence of the Blue and UV Emission Intensity. Since both the blue and W emissions could be excited at similar signal levels by exciting at 33 131 cm-' (the v7 band in the W excitation spectrum) the ratio of intensities of these two emissions (monitored at 445 and 330 nm, respectively) could be used to measure the relative amounts of I and I1 present in the jet. If the degree of freedom associated with rotamer interconversion is not significantly coupled to the expansion, the relative amounts of the two species in the jet would reflect the equilibrium
204
Heimbrook et al.
The Journal of Physical Chemlstty, Vol. 87, No. 2, 7983
c
-
32000 32250 32500 32750 33000 33250 33500 33750 34000 34250 34500 FREQUENCY ( c m - l 1 !!
F b
I
I1 I
I
I
32000 32250 32500 32750 33000 33250 33500 33750 34000 34250 34500 FREQUENCY ( c m - l 1 Figure 5. (a) ExcMon spectrum of the W emlsskn of methyl salicylate4 in a free jet. (b) Exdtatlon spectrum of the UV emission of an isotopically mixed sample of methyl salicylated and methyl salicylateh in a free Jet. Additional details are given in the text.
1
185@0
1
1
1
1
1
1
23500
1
1
1
1
/
1
/
1
/
1
1
28500
~
1
1
1
1
1
1
1
33500
FREQUENCY ( c m - l 1 Figure 6. UV emission spectrum of methyl salicylateh in a free Jet, excited at a sharp vibronic band at 33 131 cm-' (Y, of Table 111) in the excitation spectrum of this emission. The sharp features at 33 131 cm-' is scattered laser Ilght. This spectrum was obtained at 40-A resolution and was nut corrected for detector response.
established in the high-temperature reservoir. Figure 7 shows a plot of (IW/Iblue) vs. T, the ratio of 330-nm to 445-nm emission intensities vs. the reservoir temperature, for excitation at 33 131 cm-l. The data are well described by an exponential in -AHjRT. Under the usual assumption that the enthalpy difference between the two hydrogen-bonded isomers is independent of temperature, the enthalpy change associated with the reaction I I1 in the ground state near room temperature can be determined. A least-squares fit of the data of Figure 7 gives AH = 10.5 (f2.7) kJ mol-'.
-
3. Discussion 3.1. Ground-State Species and the Dual Fluorescence.
The emission spectra, shown in Figures 3 and 6, as well as the temperature dependence of the relative intensities of the UV and blue components, indicate that there are two thermally equilibrated ground-state species responsible
Tempe
atu - e ( K 1
Figure 7. The ratlo of the emission intensity at 330 nm to that at 445 nm for excitation at 33 131 cm-' vs. reservoir temperature. The line Is the function 15 exp(-lO.5 kJ mol-'lRT).
for the two different emission Components. Similar conclusions have been drawn recently by several other investigator~.'-'~ The emission maxima observed in the jet for these components (335 and 440 nm) are in reasonable agreement with recently published values for the room temperature gas-phase emission maxima.&l' We conclude that rotamers I and I1 of methyl salicylate are the species responsible for the blue and UV emission components observed in the jet. The appearance of well-resolved spectra for both species in the low-temperature gas phase contrasts with low-temperature condensed phase work where only the blue emitting species is The relative concentration of these species appears to be determined by the reservoir temperature. The temperature dependence of these two emission components in the jet gives an estimate of the enthalpy differences between the two ground-state forms (AH = 10.5 f 2.7 kJ mol-') which is comparable to previous determinations: 11.9 f 3.0,19 10.5,n 16.8,'O and 15 kJ The values of 11.9 and 10.5
Singlet State of Methyl Salicylate
2
The Joumal of Physical Chemistty, Vol. 87, No. 2, 1983 285
TABLE I : Vibrational Analysis of the Major Bands in the Excitation Spectrum of the Blue Emission of Methyl Salicylate in a Free Jeta methyl salicylate-h methyl salicylated excitation shift assignment excitation shift assignment
16 17
3 0 052.3 30 058.2 30 228.2 30 235.5 30 399.1 3 0 405.6 30424.1 3 0 428.8 3 0 476.1 3 0 484.4 3 0 573.0 30 582.3 3 0 587.0 30 621.7 3 0 629.2 3 0 647.0 30 651.7
0 6 1 76 183 34 7 353 372 377 424 432 52 1 530 535 569 577 595 599
18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
3 0 746.0 30 751.7 30 758.3 3 0 769.6 3 0 773.4 30 791.4 3 0 822.3 3 0 828.5 3 0 834.4 3 0 862.7 3 0 899.0 3 0 921.0 3 0 927.7 3 0 940.1 3 0 947.8 30 956.4 30 960.2 30 965.0 3 0 971.7 30 984.2 30 999.6 3 1 008.2 3 1 022.7 3 1 027.5 3 1 039.0 3 1 051.6 3 1 074.8 3 1 095.0 3 1 101.8 3 1 131.8 3 1 146.4
694 699 706 7 17 721 739 770 776 782 810 847 869 875 888 896 904 90 8 913 919 932 947 956 970 97 5 987 999 1022 1043 1050 1080 1094
49 50
3 1 192.1 3 1 216.4
1140 1164
1 2 3 4 5 6 7 8 9
10 11 12 13 14 15
0-0 v o or van der Waals He complex v1
vo + v1
v,)
v1
+ v,; 2v,
v 11 v1
+ 2v4 2v1 + 2v,;3v1 6v1:4v, + v 2 VI + v l J ; v 1 + v l o ; v 2 + v 9 (most intense H band) 2v,; v 2 + v 3 + v 4 2v, t v,: v 4 + v,: v , + v, v9
1
30 151.1
0
2 3 4
30 336.9 30 34 1.5 30 457.0
186 190 306
5 6 7
30 518.4 30 548.2 30 566.0
367 397 415
8 9 10
30 686.1 3 0 732.3 30 739.9
53 5 58 1 589
11
30 803.3
652
12
30 888.9
737
13
30 968.4
817
14
3 1 056.9
905
15
3 1 076.2
92 5
16 17
3 1 103.3 3 1 124.6
952 973
18
3 1 153.7
1003
v' 1 2
19
3 1 233.5
1082
VIl
20
3 1 316.6
1165
V ' 10
v
a
3 1 241.8 3 1 281.8 3 1 331.8 3 1 368.2 3 1 384.0 3 1 448.2 3 1 457.1
1189 1230 1280 1316 1332 1396 1405
+
VI6
+
v'9
+ v',;i, + + v',
2v', v',
51 52 53 54 55 56 57
11
VIg
VI6
2v6
7~1 v3
+
v9
+ 2v,; v 2 + v 3 + v, v 1 + v 2 + V 8 v, + v,; v 4 + v 9 4v1 + 2v2 8 v l ; v6 + v 8 VI
The estimated accuracy is i 2 cm-I.
kJ mol-' were based on ultrasonic relaxation measurements in liquid methyl salicylate while the values of 16.8 and 15 kJ mol-' were estimated from the temperature dependence of fluorescence intensity in static methyl salicylate gas. (21) Weak lower energy peaks observed below the 30052.3-cm-' band are now believed due to trace impurity compound(s) or complexed molecules in the methyl salicylate. (22) T. Yamnaga, N. Tatsumoto, and M. Miura, J.Phys. Chem., 73, 477 (1969).
The observation that the internal rotation responsible for the interconversion between I and I1 is not effectively cooled under the conditions of the free jet expansion suggests a barrier of at least several times kT for this process. A recent meas~rement'~ of the barrier to conversion of II to I gave an activation enthalpy of 23 kJ mol-'. From the striking sharpening of the jet excitation spectra relative to those obtained in the room temperature gas phase4 and the absence of any significant hot band structure, it is clear that the other rotational and vibra-
286
Heimbrook et al.
The Journal of Physical Chemistry, Vol. 87, No. 2, 1983
TABLE 11: Lifetimes of t h e Blue Emission of Methyl Salicylate in a Free Jet as a Function of Excitation Energy methyl salicylate-h methyl salicylated excitation, cm-I
lifetime, ns
30052 30228 30399 30406 30476 30622 30647 30822
11.3 (0.1) 0-0 7 "1 8 . 9 (0.1) 9 . 5 ( 0 . 1 ) 2;' 9 . 8 ( 0 . 1 ) 1m4 8.7 (0.1) v i 9.1 (0.1) 1 z 6 7.3 ( 0 . 1 )
mode (Table I )
lifetime, ns
30 151 18.4 (0.2)
11,
f
excitation, cm-I
vu
+
30491
16.7 (0.1)
3 0 566
14.8 ( 0 . 4 )
30 740
15.7 (0.1)
1)'
uI
+
1 1 ~
tional degrees of freedom are effectively cooled in the free jet expansion. 3.2. The Blue Emission of Methyl Salicylate. The excitation spectrum of the blue emission of methyl salicylate-h (Figure 1 and Table I) has an intense origin at 30 052.3 cm-'. We assign this feature to the 0-0 of the So to SItransition of I. The air wavelength of this feature (3326.5 A) is similar to the origin wavelength (3337 A) previously observed in the excitation and emission spectra of this molecule in a Ne matrix at 4.2 K.5 In contrast to that work, the intensity of the origin in the jet excitation spectrum is nearly as high as it is for bands at the maximum of the excitation spectrum. The lifetimes observed for the blue fluorescence decrease with increasing excitation energy (section 2.2.1.3) implying a corresponding decrease in the emission yield. This could account for no more than a 30% decrease in the intensity of the jet excitation spectrum 770 cm-' above the origin relative to the
true absorption spectrum. Thus, this difference between the Ne matrix and jet excitation spectra is most probably due to a distortion of the apparent intensities in the Ne matrix by overlap of the inhomogeneously broadened lines. The dense excitation spectrum that is observed can be almost completely assigned with combinations and overtones of only 12 or 13 modes (Table I). While such an assignment is not unique, it should give a good account of the low frequency fundamentals. The origin and many of the strong vibronic lines show a satellite approximately 6 cm-l to higher energy. While this shift is about the same as that exhibited by the methyl salicylate-h:He van der Waals complex (Figure 2), its intensity does not depend on reservoir pressure as would be expected for such an assignment. The fact that it is seen so prominently on only some of the vibronic bands in Figure 1 (see Table I) also suggests that it must have another source. One possibility is that this splitting has its origin in the hindered rotation of the methoxy group. That is, that it is due to the existence of two different but nearly isoenergetic conformers, both with the hydrogen bonding of form I but with the methyl either cis or trans to the carbonyl group. One striking feature of the methyl salicylate-h blue excitation spectrum is the presence of a low-frequency mode (176 cm-l) which combines with other fundamentals and also exhibits a rather long harmonic progression built on the origin. This low-frequency mode is probably a mode of the ring which contains the intramolecular hydrogen bond although the absence of a significant shift upon deuteration rules out the so-called hydrogen-bond bend. The 347-cm-' mode (v2 of Table I) which also is found in various combinations and overtones is roughly the same
TABLE 111: Vibrational Analysis of t h e Excitation Spectrum of t h e Ultraviolet Emission of Methyl Salicylate in a Free Jeta methyl salicylate-h methyl salicylated excitation, cm-'
shift, cm-'
6
32 303 32 447 32 475 32 483 32 490
0 144 172 180 187
6 7 8
3 2 626 3 2 667 32 691
323 364 388
9
501 5 08 522 539
17 18 19 20
3 2 804 32 811 32 8 2 5 3 2 842 32 8 8 0 3 3 026 3 3 048 3 3 076 33 131 3 3 229 3 3 295 3 3 567
723 745 772 828 926 992 1264
'I m
21
3 3 709
14 06
v1
+
"8
"3
+
'8
1
2 3 4
10 11
12 13
14 15 16
a
22 3 3 744 1441 23 3 3 752 1449 24 3 3 875 1572 25 33 8 8 9 1586 26 3 3 982 1679 27 3 4 310 2007 28 34 396 2093 The estimated accuracy is + 3 cm-'
excitation, cm-'
assignment 0-0 vi
"2
"3 'I
4
"1
1J3
Iil
?-
+ +
+
v3
shift, cm-'
1 2 3 4 5 6 7
32 324 32 471 3 2 495 32 504 32 510 3 2 573 3 2 638
0 147 171 180 186 249 314
8 9 10 11
32 716 3 2 759 32 799 32817
12
assignment 0-0 1' I 2 " 3 " 4
11' I
+
392 435 475 493
v',
+
32 868
544
1)'
13 14 15 16 17 18 19 20 21
33 068 3 3 091 33 145 3 3 245 3 3 319 3 3 594 (33 6 0 2 ) 3 3 748 (33 7 5 8 )
744 767 821 921 996 1270 (1278) 1424 (1434)
22 23 24
34 006 34 340 34 417
1682 2016 2093
11'
v4
2", 113
+
"I2
v4
2v2 + u 3 "5
~
v 4 t vi '6
['7 "3
v4
+ 6'
+
1J6
+ + +
"1
+
V6
VI
vm v, 113
V8
V8
+
'6
" 1
+ +
v'6 v',
(y'd
,
(Utl
I"g)
Vl3
vi2 " 8
Um
v
6
+
",a
V I + V E
+
vi3
Singlet State of Methyl Salicylate
as one observed to form a progression in the excitation spectrum observed by Brus and Goodman5at 4.2 K in the condensed phase. However, what was taken for increased intensity of transitions involving higher numbers of quanta of this mode may have instead been due to an increased density of vibronic lines in the inhomogeneously broadened spectrum. This 347-cm-' mode is of nearly the same frequency as the benzene v6,-like band in benzoic acid hydrogen-bonded dimer which has been observed at 335 cm-1.23 In the case of the benzoic acid dimer, the other half of the doubly degenerate benzene v6 el, mode (v&) is found at 557 The bands at 569 and 595 cm-' are equally likely candidates for this mode in methyl salicylate-h. In a recently published free jet excitation spectra of phenol the v6a band in uncomplexed phenol was observed at 490 cm-1.25 The vg and vl0 modes at 904 and 919 cm-' (Table I) are likewise candidates for the benzene v12-likemode found at similar frequencies in both phenol and benzoic acid s p e ~ t r a . ~The " ~ ~strongest peak in the methyl salicylate-h blue excitation spectrum occurs at 1094 cm-'. While this peak can be assigned as a combinationband, it seems more likely that a significant fraction of the intensity here is due to a new fundamental, v i 3 of Table I. It is possible to associate this mode with the carbonyl stretch frequency, considerably reduced in the excited state. Such an identification is consistent with hydroxy proton translocation toward the carbonyl oxygen, concomitant reduction of the double bond character of the carbonyl carbon-oxygen bond, and hence, reduction of the carbonyl stretch force constant. The vibrational development of this transition is consistent with the assignment of the upper level to a a-a state. This assignment is supported by the intensity of this transition in both the condensed and gas phases. Integrating the published solution absorption spectrums gives f = 0.062. This oscillator strength is consistent with estimates of the radiative lifetime in hydrocarbon solution of 15 ns (7 = 339 ps;12yield = 0.022'O) and in the gas phase of 77 ns (Strickler-Berg analysis of the gas-phase absorptions and free jet emission spectra) and agrees with the oscillator strength for the a-a state calculated by using approximate molecular orbital methods.16 This is much more intense than would be expected for a transition to an n-a* level. Monodeuteration of methyl salicylate results in a 99-cm-' blue shift of the origin of the excitation spectrum. The vibrational shifts of the more intense peaks in the spectrum of methyl salicylated (340 (v2,),415 (vqr), 535 (v5,),and 589 cm-' (vet)) are readily correlated with the analogous modes in the methyl salicylate-h spectrum (Table I). The measured deuteration shift is essentially identical with the one found in the condensed phase.5 Normally, monodeuteration would not be expected to produce such a large isotope shift in the origin. For example, in the benzoic acid dimer monodeuteration of the carboxylic proton results in only a 23-cm-I blue shift of the origin of the first excited a-a* state.24 In the case of methyl salicylate it has generally been assumed that in the excited state there is a significant shift of the hydroxyl proton along the 0-H stretch coordinate toward the carbonyl oxygen. This picture accounts for the large Stokes shift in the blue fluorescence as discussed in (23)J. C. Baum and D. S. McClure, J. Am. Chem. SOC.,101, 2335 (1979). (24)J. C. Baum and D. S. McClure, J. Am. Chem. SOC.,102,720 (1980). (25)H.Abe, N.Mikami, and M. Ito, J.Phys. Chem., 86,1768(1982).
The Journal of Physical Chemistry, Vol. 87,No. 2, 7983 287
section 1. It is reasonable to expect a reduction in the excited state of the 0-H stretch force constant to accompany this displacement. Taking the harmonic ground state 0-H stretch frequency as 3258 cm-1,26assuming that deuteration roughly reduces the ground and excited state 0-H stretch frequencies by (2)-1/2,27and ascribing most of the deuteration shift to differences in the zero point energies of this mode in the ground and excited states, we estimate the excited state 0-H stretch frequency to be 2582 cm-'. If we use this frequency and the recently determined correlation between hydrogen-bond strength and 0-H stretch frequency,28the hydrogen-bond strength in the excited state would be 60 kJ mol-', about double the ground state v a l ~ e . ' ~ ~ ~ ~ The proton displacement hypothesis is further substantiated by the red shift in the fluorescence maximum of the blue emission of methyl salicylate-h that occurs upon increasing the excitation energy (Figure 3 and section 2.2.1.2.). Such a result is expected if there is a significant coordinate displacement and no energy relaxation prior to emission. There can be little doubt that proton translocation occurs in the excited state. The remaining questions regarding the photophysics of this process in methyl salicylate revolve around the extent of the proton translocation and the magnitude of the barrier to this process. It is well-known that the existence of a barrier to proton transfer can cause a doubling of the 0-H stretch energy levels. The calculation of eigenstates and eigenvalues for the general quartic potential that is appropriate to this case can be done straightfor~ardly.~~ Recently, there have been several studies in which spectroscopic results have been used to determine the double minimum potential parameters in ground and excited electronic states.3w32 In the case of methyl salicylate, we are unable to identify any bands that could be attributed to the 0-H stretch coordinate. This is primarily due to the onset of a nonradiative decay channel at approximately 1500 cm-' above the 0-0. While this decay precludes the possibility of observing excitation features in the region of the 0-H or 0-D stretch fundamental, the possibility that low-frequency features associated with the anharmonicity of the potential could be seen and identified by their deuteration shift remains. Unfortunately, all of the features that can unambiguously be associated with methyl salicylated are only slightly shifted from their methyl salicylate-h counterparts. None the less, an estimate of the distance of proton transfer can be made based on the deuteration shift of the 0-0. The shift of 99 cm-I suggests an excited-state 0-H stretch frequency of 2582 cm-l as described above. The correlation between 0-H stretch frequency and the change in bond length33 suggests that the proton has moved 0.1-0.2 A closer to the carbonyl oxygen in the excited state of rotamer I. This is less than would be expected for a symmetrical hydrogen bonda5J7 The lifetimes of the blue emission of methyl salicylate-h and methyl salicylated shown in Table I1 contain several (26)J. Stevenson, private communication. (27)Actually deuteration probably reduces the 0-H stretch frequency by only (1.35)-1. See, e.g., E. Czarnecka and A. Tramer, Acta Phys. Polon., 36,133 (1969). (28)M. Takasuka and Y. Matsui, J.Chem. Soc., Perkin Trans., 1743 (1979). (29)R. L. Somorjai and D. F. Hornig, J. Chem. Phys., 36,1980(1962). (30)R. Rossetti and L. E. Brus. J. Chem. Phvs.. 73. 1547 (1980). (31)R. Rossetti, R.C. Haddon, k d L. E.Brus;J.'A&. Chem. SOC., 102., 6913 - - ~ (1980). ~ - ~ - - - , (32)R. Rossetti, R.Rayford, R. C. Haddon, and L. E.Brus, J. Am. Chem. SOC.,103,4303 (1981). (33)H. Ratajczak and W. J. Orville-Thomas, J. Mol. S t r u t . , 1, 449 (1967-68).
208
The Journal of Physical Chemistfy, Vol. 87,No. 2, 1983
interesting features. First, the methyl salicylate-h and methyl salicylate-d 0-0 lifetimes are roughly the same as the corresponding lifetimes measured in the condensed phase.5 The blue fluorescence lifetimes in both of the isotopically substituted methyl salicylates, isolated in the free jet, decline with increasing vibrational energy (Table 11). Excitation wavelength dependent quantum yieldsl0P1l and nonexponential decay" of the blue emission in the room temperature gas have already been reported. In the room temperature gas, the longest lifetime component observed (via hot band excitation at 335 nm) was 9 ns. The decrease in the blue fluorescence lifetime with increasing vibrational energy is attributable to an increased rate of nonradiative decay to a dark state of the molecule. Candidates for this process include internal conversion to another excited singlet state or a high lying vibrational level of the ground state (perhaps induced by large-amplitude out-of-plane vibration in the ring containing the hydrogen bond") or intersystem crossing to a triplet level. It seems unlikely that out-of-plane vibrational modes which prevent proton translocation can be responsible for the fluorescence quenching as previously speculated." The potential for rotamer interconversion would be expected to largely determine these modes. From the ground-state enthalpy difference of 10.5 kJ mol-' and the fact that the 0-0 of rotamer I is 2917 cm-' lower in energy than the 0-0 of rotamer 11, it follows that the excited-state enthalpy difference between the two rotamers is 3250 cm-' or 39 kJ mol-l with rotamer I again the more stable species. The existence of distinct emission spectra implies a barrier for the conversion of I1 to I in the excited state. Thus, rotamer I is bound by more than 3250 cm-' in the excited state. A minimum of this depth is incompatable with a large amplitude of vibration in the ring puckering modes at the energy of the onset of the radiationless relaxation. A vibrational energy of 700-1500 cm-l corresponds to only a few quanta of such a mode and is still well below the barrier. Further, the rate of the radiationless process correlates only with the amount of vibrational energy in the excited state rather than the excitation of any particular low-frequency mode. It is more likely that strong coupling between excited states is responsible for the fluorescence quenching a t higher vibrational levels. The assignment of the upper state involved in the blue fluorescence to a a-a* state that intersects either a singlet or triplet n-a* state a t higher energies would be consistent with all of our measurements. During the final stages of the preparation of this paper, we learned of studies of the fluorescence kinetics and dispersed emission of methyl salicylate-h in a free jet by another These studies extend the fluorescence lifetime determinations to higher excess energy and, hence, into the sub-nanosecond decay range. Where the data overlap there is quantitative agreement with our observations. Their more highly resolved 0-0 excited dispersed emission spectrum33also agrees with our own determination (Figure 3a). 3.3. The Ultraviolet Emission of Methyl Salicylate. The vibronic development of the excitation spectrum of the ultraviolet emission of methyl salicylate-h and methyl salicylated is much less complex than that of the blue emission. The spectra indicate that the potential of I1 changes very little upon excitation and has a single min(34) A. Duben, L. Goodman, and M. Koyanagi, Excited States, 1, 295 (1974). (35) M. Stockburger, Z. Phys. Chem. (Frankfurt am Main), 31, 350 (1962). (36) P. M. Felker, W. R. Lambert, and A. H. Zewail, J . Chem. Phys.,
in press.
Heimbrook et al.
imum for the proton coordinate. For methyl salicylate-h (Figure 5a and Table 11), there are several vibronic bands that can be assigned as fundamentals. The low-frequency modes (144 (ul), 172 ( u p ) , 180 (us) and 187 cm-l (v4)) are likely due to ring puckering or methyl torsions. These modes have almost exact counterparts in the ground to n-a* state absorption spectrum of vapor-phase benzaldehyde.34 The assignment of one of these modes to an -O--HO- stretch was rejected because of the lack of any significant shifts on deuteration. Three intermediate frequency modes (539 (u6), 745 (us), and 828 cm-' (u,)) are most likely associated with the aromatic benzene ring. Again, the ground to n-a* state absorption spectrum of benzaldehyde had significant intensity in a mode at 745 The highest frequency mode (u8 at 1264 cm-') is characteristic of the n-a* C=O stretch frequency for an aromatic carbonyl. Its prominence is consistent with this assignment. In methyl salicylated this band splits. Splitting of the carbonyl stretch has been observed in the ground state of the similar salicylic acid molecule,37although in that case the frequency shifted 5-20 cm-' to lower energy upon deuteration. Because the vibrational development of the UV excitation spectrum is strikingly different from that of the blue excitation spectrum and is nearly identical with that observed for the n-a* transition in benzaldehyde vapor, we assign the upper level in the UV transition as n-a*. The observed deuteration shift in the excited state origin (21 cm-') of the UV-emitting species is nearly the same as that reported for the benzoic acid dimer (23 ~ m - ' ) and ,~~ is much smaller than that of the blue-emitting species. Further evidence that the changes upon excitation of the geometries and force constants of the UV species are small and that vibrational relaxation is not significant is the observation that the emission spectrum shifts in a nearly resonant manner with the excitation wavelength. This is why it was necessary to frequently shift the UV-cutoff filters in order to observe the higher frequency bands. Given the conditions which exist in the free jet experiment and the sharpness and simplicity of the excitation spectrum of the UV emission, it is clear that this species must have structure I1 shown in Figure 1. Structures with broken bonds would be expected to give a more diffuse spectrum. Although an attempt was made to measure the fluorescence lifetime of the UV-emitting species (II), the observed fluorescence was so short-lived and the signal level so low that the decay kinetics could not be reliably deconvolved from the 6-11s excitation pulse. We estimate the lifetime of this state to be 2 ns or less for both methyl salicylate-h and methyl salilcylate-d. This is consistent with the kinetics of the UV fluorescence of the methyl salicylate-h in a room temperature gas which indicated that the longest lifetime component was on the order of 1.5 ns." This is short for the radiative lifetime of an n-a* state, but would be consistent with a low quantum yield for emission resulting from rapid intersystem crossing from the n-a* singlet to a lower lying a-a* triplet level. The vapor-phase quantum yield for the n-a* fluorescence of benzaldehyde has been determined to be less than 10-3.35 The quantum yield for the UV emission of methyl salicylate has been estimate to be 0.271° which is clearly incompatible with an n-a* assignment. Until a direct determination of the emission yield of the UV species is made, it is not clear whether the lifetime of the SI state (37) J. M. Friedman, D.L. Rousseau, and C. Shen, Spectrochim. Acta, Part A , 35, 989 (1979).
J. Phys. Chem. 1983, 87,289-293
of I1 is controlled by radiative or nonradiative processes. The small energy gap between n--P* and T-P* states in similar molecule^'^-^^ could also enhance both the radiative and nonradiative decay of the n--P* state. 4. Summary
The dual fluorescence exhibited by methyl salicylate in solution and the room temperature vapor phase can also be observed in the low-temperature gas phase as realized when methyl salicylate is seeded in a supersonic helium jet. Under these conditions it is possible to measure highly resolved excitation spectra for each of the emission components of both methyl salilcylate-h and methyl salicylate-d. The excitation spectrum of the blue emission becomes weak and diffuse roughly 1500 cm-’ above the 0-0 which is at 30052.3 cm-l. This is associated with the onset of an efficient radiationless decay as is verified by the monotonic decrease of emission decay time with increasing excitation energy. From our analysis of the excitation spectra, emission spectra, and fluorescence decay kinetics as a function of excitation energy we conclude the following: (1) the dual emission comes from two different internally hydrogen-bonded ground-state species. (2) The blue emission component derives from the species where the internal hydrogen bond is between the phenol and carbonyl groups. (a) This is the more stable ground-state species by 10.5 kJ mole-’. (b) The excited state is most likely a a--P* state in which the phenolic proton is significantly displaced toward the carbonyl oxygen. (c) Roughly 1500 cm-’ above the 0-0 this T-T* state is strongly coupled to a radiationless decay channel, per-
Electronic Structure of H,
+
and HeH,
+
289
haps involving internal conversion to an n--P* singlet or n--P* triplet level. (d) Because of the onset of this radiationless process, it is not possible to locate any bands associated with the O-.H-O stretch mode. Thus, it is not possible to fix more quantitatively the form of the hydrogen-bond potential in this particular case. (3) The W emission component derives from the species where the internal hydrogen bond is between the phenol and methoxy groups. In this case the excited state is most likely an n-a* state in which there is very little displacement of the phenolic proton from its ground-state equilibrium position. While the close proximity of the n-a* and -P-a* states in methyl salicylate provides the opportunity to examine in detail some extremely interesting coupling and relaxation behavior, the attendant spectroscopic complications preclude the possibility of simply and unambiguously fixing the hydrogen-bond potential. Acknowledgment. Ricia Noyes has valiantly types this manuscript many times on many different machines. We have benefitted from discussions with Professor Gregory Gillispie and also thank Professor A. H. Zewail for making a preprint of his communication on methyl salicylate available to us prior to its publication. This work was supported in part by grants from the National Science Foundation (CHE-7809444) and the National Institutes of Health (EY-01733). Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society. Registry No. Methyl salicylate-h,119-36-8;methyl salicylated, 83719-87-3.
Clusters
Susanne Raynor and Dudley R. Herschbach’ Department of Chemistry, Harvard University, Cambridge, Massachusetts 02 138 (Received:August 6, 1982; I n flnal form: September 15, 1982)
The “solvation”of a H3+ molecule-ion by H2 molecule(s) and/or a He atom was examined in ab initio electronic structure calculations. Binding energies and geometrical parameters were determined for the ground electronic states of H,’ (n = 3 , 5 , 7 , 9 , 1 1 ) and HeH,+ (n = 3 , 5 , 7 , 9 ) cluster ions. Floating spherical Slater orbitals (FSSO) were employed with full single and double configuration interaction (CI). All of the clusters studied were found to be weakly bound (with respect to removal of “solvent”species). Inclusion of configuration interaction had a major effect; for n 2 5, it increased the energy required to remove an H2molecule from H,+ by -35% and that to remove the He atom from HeH,+ by -300-400%. The net solvation energy per H2 (-3.5 kcal mol-’ for the first three solute molecules, 1.5 kcal mol-l for the fourth) was found to be about threefold larger than for He.
-
Introduction The H3+system is extremely stable; it thus appears as the dominant product in a large class of ion-molecule reactions2 and also provides the core cation for a prototypical Rydberg polyatomic m ~ l e c u l e . ~ Previous -~ ab in-
’
(1) Salmon, L.; Poshusta, R. D. J. Chem. Phys. 1973,59, 3497. (2) Porter, R. F. In “Interaction Between Ions and Molecules”;Ausloos, P., Ed.; Plenum Press: New York, 1975; p 231. Bohme, D. K. Ibid, p 489. (3) Herzberg, G. Discuss. Faraday SOC.1981, 71,165 and references
therein. (4) King, H. F.; Morokuma, K. J. Chem. Phys. 1979, 71, 3213.
0022-365418312087-0289$01 .SO10
itio calculations have shown that cluster ions of the form H3+(H2), are weakly stable, in agreement with experimental observations.610 These complexes correspond to “solvation”of a central H3+ion by H2 molecules, as shown (5) Raynor, S.; Herschbach, D. R. J. Phys. Chem., 1982,86,1214,3952. (6) Clampitt, R.; Gowland, L. Nature (London) 1969, 223, 815. (7) Hiraoka, K.; Kebarle, P. J. Chem. Phys. 1975, 62, 2267. (8) Arifov, U. A.; Pozharov, S. L.; Chernov, I. G.; Mukhamediev, 2. A. High Energy Chem. (Engl. Transl.) 1971,5, 69, 79. (9) Bennett, S. L.; Field, F. H. J. Am. Chem. SOC.1972, 94, 8669. (IO)Johnsen, R.; Huang, C. M.; Biondi, M. A. J . Chem. Phys. 1976, 65, 1539.
0 1983 American Chemical Society
