1244
J. Php. Chem. 1982, 86, 1244-1247
other radicals and products, etc.). When reactants are present, the benzyl disappearance is clearly first order and is fit well by a single exponential over the 2-3 reaction lifetimes observed.
Results and Discussion Second-order rate constants for the reactions of benzyl radicals with O2and C12were obtained at three temperatures by measuring the pseudo-first-order reaction rate at these temperatures as a function of O2or C12pressure. The data at room temperature are shown graphically in Figure 1and the resulting rate constantsat all three temperatures are listed in Table I. The pseudo-first-order benzyl disappearance rates are computer fit by linear least-squares regression analysis (200-1000 data points). The secondorder rates reported in Table I are least-squaresfits to the data. All uncertainties are f l a confidence limits. Within the precision of the measurements, the rate constant for benzyl + O2appears to be independent of temperature over the range studied. The average of the three determinations, (1.05 f 0.12) X 1@12cm3molecule-' s-l, is in excellent agreement with the resulta reported by Ebata, Obi, and Tan&? (0.99 f 0.07)X 10-l2cm3 molecule-' s-l. Over the preasure range available in this study, 2-15 torr, we observe no pressure dependence of the measured bimolecular rate constant which confirms the lack of pressure dependence observed by Ebata,Obi, and Tanakagover the much wider range of pressures used in their study. They conclude that the excess energy which results from bond formation in the peroxide complex (13 kcal mol-l according to a calculation by Benson") is rapidly internally redistributed into the many modes available. As noted above, we observe no temperature dependence of the measured rate constant although we are not able to exclude a slight decrease with temperature. The rate constant for the reaction of benzyl with C12, however, shows a slight increase with temperature. An Arrhenius plot of these data, Figure 2, yields an activation energy of 880 f 230 cal/mol. The rate constant derived (11) S.W. Benson, J. Am. Chem. Soc., 87, 972 (1966).
26
20
30
32
34
lOOO/T
F l g w 2. The Arrhenius pkt for data on the reactkn of benzyl radical with C&. Thelhempmentsa llnearleast-squaresfit tothedata which
are marked with wror
bars representing f2a.
is (5.7 f 1.9) X 10-l2exp(-880 f 230/RT). No pressure dependence was observed over the range of 2-15 torr total pressure. The observation of the slight activation energy is consistent with this being an atom transfer reaction. In order to investigate the possibility of condensation reactions with unsaturated hydrocarbons, benzyl radical reactions with cis-2-butene, methylacetylene, and allene were studied at 295 K. In all cases, no reaction to our limit of detection, cm3 molecule-' s-l, was observed. In this Letter we report the following: (a) the preparation of benzyl radical by laser photodissociation of benzyl chloride at 249 nm; (b) the detection of vibrationally and rotationally thermalized benzyl radicals by laser-induced fluorescence at 448 nm; (c) the rate constant for the addition reaction of benzyl radical with molecular oxygen; (d) the temperature-dependent rate for the apparent abstraction reaction of benzyl with molecular chlorine; and (e) upper limits for the reaction rate of benzyl with several types of unsaturated hydrocarbons. The reactivity of the benzyl radical has been investigated and is discussed within the context of ita potential importance as a flame radical in combustion processes.
Evidence for Mode-SpecCfic I ntramoiecuiar Vibratlonai Redistribution in S, p -DHluorobenzene Scott H. KaMe, Warren D. Lawrance, and Alan E. W. Knight' School of Science, &.rmth Unlwrdty, Nathan, &Isbane, Queens&nd, Awtrak, 4 11 1 (RecelW: December 15, 196 1)
The extent of intramolecular vibrational redistribution (IVR) occurring within the fluorescence lifetime of SI p-difluorobenzenehas been measured with single vibronic level fluorescence spectroscopy for the levels Oo, 6l, 5l, 5'301, 51302,3l, El2, 52301,52302,and 3l5l. We find that Sl levels involving one or more quanta of excitation in va undergo IVR to a greater extent than thw with no excitation in vy). We conclude that vibrational excitation in the out-of-plane mode v m enhances IVR and we postulate that this may be a consequence of a pseudo-double minimum potential associated with displacement in vs0.
The rate at which a laser-pumped vibrational mode dissipates its energy to other modes in a molecule is an important factor in determining the potential for modeselective, laser-induced chemistry in polyatomics. In this 0022-3654/82/2086-1244$01.25/0
context, the central question to be addressed for unimolecular reactions is whether or not the process of intramolecular vibrational redistribution (IVR) displays any sensitivity to the vibrational mode initially excited. 0 1982 American Chemical Society
Letters
The Journal of phvslcal Chemistry, Vol. 86, No. 8, 1982 1245
Parmenter and co-workers' have used single vibronic level (SVL)fluorescence spectroscopy to demonstrate that the onset of intramolecular vibrational redistribution in the S1 state of p-difluorobenzene (pDFB) is dependent on the total SIvibrational energy content of the excited vibronic state. The extent of redistribution is gauged by measuring the ratio of unstructured to structured emission in the SVL fluorescence spectrum. Spectral congestion in fluorescence increases with increased vibrational excitation and essentially no structured emission can be observed from pDFB when excitation encounters levels with more than say 3000 cm-l of excess vibrational energy. The diffuse spectrum is considered to be the manifestation of complete vibrational redistribution occurring on a time scale much faster than that of fluorescence emission (ca. 10 ns in pDFB). Similar results have been observed for other polyatomics such as azulene? coumarone: l-azaindolizine: and the alkylben~enes.~In the longer alkylbenzenes and in the phenoxyalkanes6 the onset of IVR gauged by the appearance of a broad, red-shifted background in the SVL fluorescence spectrum, was seen to occur relatively low in the S1vibrational manifold (EvibN 530 cm-I). In this Letter, we present evidence which suggests that the onset of intramolecular vibrational redistribution in the S1state of p-difluorobenzene may be dependent not only on exem vibrational energy content but also on which vibrational mode carries the initial excitation. We have used SVL fluorescence to probe the IVR process. Our attention has been directed toward establishing whether fluorescence from any particular class of vibronic levels displays significantly greater unstructured emission than that which may be expected on the grounds of vibrational energy content alone. The SI So absorption spectrum of pDFB possesses particular features which have facilitated our study. Each of the dominant progressions in the spectrum show strong sequences in the low frequency, out-of-planemode vm (Jm cv 122 ~m-~)?,'Excitation of these sequence bands can be achieved fairly selectively with narrow-band (52 cm-9 laser excitation. We have chosen to explore the SVL fluorescence spectra obtained after excitation of the progression members 5&and 52 and their accompanying sequences 5;30:, 5&30:,and 5!30:, 5i30:. Tunable excitation was provided by frequency doubling the output of an amplified dye laser pumped by a pulsed Nd:YAG laser. Fluorescence from a simple cross-shaped sample cell was dispersed with a 2.5-m cosecant scanning Czerny-Turner spectrometer constructed in our laboratories. Detection consisted of a high-gain photomultiplier, boxcar averager, and chart recorder. Spectra were digitized by using a graphics peripheral to our laboratory computer, enabling areas of structured and unstructured emission to be determined with a precision of f5%. The pressure of pDFB was kept at 10 mtorr throughout the experiments. Figure 1 provides an example of the spectra obtained after selective excitation of each of the levels 52, 52301,and 52302. These three levels lie at energies 1629, 1761, and 1882 cm-', respectively, above the S1 zero-point vibrational level. In each case, the position of excitation has been
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(1) R.A. Coveleskie,D.A. Dolaon, C. S. Parmenter, and T. M. Dunn, unpublished manuscript. (2) W. D. Lawrance and A. E. W. Knight, unpublished resulta. (3)B. M. Stone and C. S. Parmenter, privata communication. (4) J. B. Hopkins, D.E. Powers, and R. E. Smalley, J . Chem. Phys., 72,2905, 5039, 5049 (1980). (5) J. B. Hopkins, D. E. Powers, and R. E. Smalley,J. Chem. Phys., 74,6986 (1981). (6)T.Cvitas and J. M.H o b , Mol. Phys., 18,793 (1970). (7)M.J. Roby and E. W. Schlag, Chem. Phys., 30,9 (1978).
5' 30'
i i
52
ik 30000
azooo
34000
seooo
38000
ENERGY cm-'
Figure 1. Single vibronic level fluorescence from the levels 5', 5*30', and 5*302in S, pdifluorobenzene. excitation positions, marked with an asterisk, are 38 469,38 436, and 38 392 cm-' (vac), respectively. The bottom trace corresponds to emission observed after excitation in a featureless regkm, 70 cm-' higher in energy than the 5; band maximum. For ail spectra the laser excitation bandwidth was 5 2 cm-' and fluorescence resolution was - 2 cm-'.
chosen carefully to coincide with the absorption band maximum which occurs near the higher energy extreme of the rotational band contour. This choice of excitation position minimizes interference due to overlap from the rotational tails of bands whose masima lie at higher energy than that for the band in question. In our calculations of the ratio of unstructured to structured emission, we have taken account of the contribution to this ratio from the rotational tail of the adjacent band (if any) lying at higher energy. The details are described elsewhere.8 The spectrum displayed at the bottom of Figure 1corresponds to the fluorescence observed after excitation into an essentially featureless region of the absorption spectrum, some 70 cm-' higher in energy than the 5: band maximum. The rationale for obtaining this spectrum is discussed below. The spectra shown in Figure 1 demonstrate that the addition of first one and then a second quantum of um to the emitting vibronic state leads to a significantly increased unstructured background emission. The unstructured emission extends over the range -1800- to -7500-cm-' displacement from excitation, with a broad maximum near -4OOO-cm-' displacement. These features of the unstructured emission are essentially the same as are observed in fluorescence from levels lying much higher in the SIvibrational manifold and which have been identified previously as displaying extensive IVR.' Moreover, we find that the addition of a high pressure (3000 kPa) of the electronic quencher O2 leads to a dramatic reduction of the congested emission intensity in the fj2301and 52302 spectra. Quenching by O2 at this pressure provides a timing "gate" which limits our observation of fluorescence to only those molecules which emit promptly, that is, within -15 ps. These observations are consistent with expectations based on the oxygen-quenching experiments conducted by Coveleskie, Dolson, and Parmenterg that (8) S. H. Kable, W. D. Lawrance, and A. E. W. Knight, manuscript in preparation.
1240
1.5
~
'
5 ' 3024
1.0 v
Letters
The Journal of Physical Chemistry, Vol. 86, No. 8, 1982
)
I
-l
'
5'30'4
'7
/
0.5
I
__
O0
0 0
0
6' A
,
500
I
1000
1500
2000
S, VIBRATIONAL ENERGY / cm-'
Figure 2. Plot showing the ratlo U/S of unstructured to structured emission (see text) observed in SVL fluorescence spectra from a variety of vlbronic levels in S, pdifluorobenrene. The abscissa glves the S1 excess vlbratlonal energy content of each level. Llnes joining points are guMellnes only.
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have established the time scale for intramolecular vibrational redistribution for the 3l5'30' level of pDFB (Eeb 2191 cm-'). Our assignment of the structured emission from the levels 52301and 52302confirms that there is negligible spectral contamination arising as a result of excitation of other levels reached through overlapping absorption. There are, however, some differences in relative band intensities in progressions involving vs and v6 which distinguish the three spectra shown in Figure 1. These differences in relative intensity arise largely from the slight variations in the nature of the anharmonic coupling which defines the vibronic character of each of the three emitting states. The dominant contribution to this anharmonic coupling comes from the Fermi resonance between the near degenerate states 5l and 62 (and hence the triad 52, 5162, 6') identified in SVL fluorescence studies.'O By varying the position of excitation a few wave numbers we have established that the vibrational structure in each of 52, 52301,and 52302SVL fluorescence follows a pattern which is consistent with that expected for the Fermi resonances involving vs' and V6'. There is a remaining issue which requires consideration before a firm conclusion can be drawn from the spectra shown in Figure 1. The absorption spectrum of pDFB, measured by using a double-beam method, shows substantial contribution to unstructured absorption in regions between and beneath prominent band maxima.' Indeed, this is a common feature, usually left unexplained, in the absorption spectra of many polyatomics, for example, benzene12and naphthalene.'&'' This diffuse and relatively constant background absorption accounts for some 15% of the absorption intensity in the region of the 5g band maximum. This percentage increases to -25% for the 5i30: band maximum. The fear that the unavoidable excitation of this diffuse, background absorption may con(9)R. A. Coveleskie, D. A. Dolson, and C. s. Parmenter, J. Chem. Phys., 72,5774 (1980). (10)R.A. Coveleskie and C. S.Parmentar,J. Mol. Spectrosc., 86,s (1981). (11)W.D. hwrance and A. E. W. Knight, unpublished rasulta; aee also an analysis of SVL fluorescence from the Fermi doublet, 6; 69,in the SIstate of pyrimidine: A. E. W. Knight, C. M. Lawburgh, and C. S. Parmenter, J. Chem. Phys., 63,4336(1975). (12)J. H.Callomon, T. M. Dunn, and I. M. Mills, Phil. Trans. R. SOC. London, Ser. A , 269,499 (1966). (13)D.P. Craig and J. M. Hollas, Phil. Tram. R . Soc. London, Ser. A , 253 569 (1961); (14)A. E.W.Knight, B. K. Selinger, and I. G. Ross,A u t . J. Chem., 26, 1159 (1973).
...
tribute significantly to unstructured emission, especially in 52302fluorescence, is dispelled by the "spectrum" at the bottom of Figure 1 and referred to earlier. The contribution is small and in any case we have subtracted it out when calculating the ratio of unstructured to structured fluorescence. Figure 2 presents a summary of the data that are obtained after making quantitative measurements of the ratio of unstructured to structured emission (U/S) for each of the SVL fluorescence spectra shown in Figure 1, as well as for the trio of levels 5', 51301, and 5'302. These measurements are compared with those obtained by us for a number of other levels in the S1state of pDFB. The other levels are distinguished by the absence of any excitation in the mode v3@ The ratio U/S is a relatively smooth function of S1vibrational energy content for the levels Oo, 6', 5') 3', 52,and 3'5'. Our spectra for these levels were obtained with improved resolution (excitation 5 2 cm-', fluorescence 2 cm-') over those kindly provided to us by Parmenter.'O Accordingly, our U/S ratios are a little less than those obtained by digitizing and integrating the spectra of Parmenter et al. since the unstructured emission is not added to by the manifestations of poorer spectral resolution. Indeed, we have established that the data shown in Figure 2 represent a secure lower limit to the U/S ratios for this excitation width.8 Whereas the U/S ratio for levels which do not involve v m lies on a smoothly rising curve, Figure 2 shows that the U/S ratio for the levels 51301,5l302, 52301,and 52302deviate significantlyupward. The deviation is most significant for 52302(Evib= 1882 cm-'). For this level, the U/S ratio is 30% greater than that for the level 3'5' which lies 187 cm-' higher in energy (Evib= 2069 cm-'). From these data, we may conclude that excitation of the low-frequency, out-of-plane mode ~ 3 0 ,in combination with excitation of v5, leads to significantly enhanced IVR in S1 pDFB relative to levels which do not involve any excitation in vw Hence the onset of intramolecular vibrational redistribution in S1pDFB appears to display a dependence on the vibrational modes which are initially excited. An explanation for the efficiency of um in promoting IVR in S1pDFB may stem from the appreciable anharmonicity seen for this mode. In the absorption spectrum, sequence bands of type 30; are unevenly displaced from the parent transition. For example, from our own high-resolution absorption measurements we find that the band maximum for 52 is at 38 467 cm-'; the sequence transitions 5g30: and 5;302B lie at -33.3 and 433.3 + 43.4) cm-', respectively. In contrast, the corresponding sequences in ~ 3 built 0 on the origin lie at -42.0 and 442.0 + 33.7) cm-l, respectively. Clearly, there are perturbations affecting v3@ Evidence from the work of Parmenter et al.'JO and our own SVL fluorescence spectra8 from the levels Oo, 301, and 302 indicates that the excited-state potential function for um may be described as a pseudo-double minimum. Accordingly, the higher order contributions to a Taylor series expansion in Qm about the potential minima must be greater than for other modes. This follows simply because a predominant quadratic term cannot provide an adequate de0 The mechanism scription of the distorted ~ 3 potential. for IVR is based upon anharmonic interactions (cubic and higher order) between the initially pumped level and those levels lying degenerate within the line width of the initial 0 promoting IVR state. The anomalous behavior of ~ 3 in may therefore arise as a result of its intrinsically greater anharmonicity. An alternative explanation is that the levels involving v m show increased IVR as a consequence of the larger total
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J. phys. Chem. 1002, 86, 1247-1250
number of quanta involved in these vibrations relative to those not involving vw However, evidence recently obtained in our laborah$ suggests that at these energies the number of quanta associated with the excited vibrational level is of lesser importance in the IVR process.
Acknowledgment. This work was supported by the
1247
Australian Research Grants Committee. We are grateful
to Professor C. S. Parmenter and his co-workers for providing us with preprints of their work prior to publication. The assistance of Messrs. I. Finch, J. Field, K. Selway, J. Taylor, R. Barrett, and S.Barrett in constructing the experimental apparatus for this work is gratefully acknowledged.
Solid-state Magic-Angle Spinning Aluminur-27 Nuclear Magnetic Resonance Studies of Zeolites Using a 400-MHz High-Resolution Spectrometer C. A. Fyfe,' 0. C. Qobbl, J. S. Hartman,' The Gudph-Waterloo Centre for 6Vaduate Wwk In Chemistry, Department of Chemistry, Unlversiiy of G d p h , Gwlph, On&& NlQ 2W1, Canada
J. Kllnowskl, and J. M. Thomas' Department of Physhl &misW, Universiiy of Cambrkjge, Cam&In Final Form: Febmty 8, 1982)
CB2 IEP, Unned Khgdwn flecelved: December 15, 1981;
High-resolution solid-state nAl NMR spectra (with magic-angle spinning) of a number of zeolites with different framework structures have been recorded at 104.22 MHz on a conventional high-field spectrometer (proton frequency 400 MHz) with a home-made spinning probe. Spectra of NazeoIites comprise one narrow peak with a chemical shift ranging, in different materials, from 51.5 to 65.0 ppm from [A1(HzO)6]3+.In dealuminated zeolites, an additional peak is observed corresponding to octahedrally coordinated aluminum in the zeolitic channels. 27AlNMR is most valuable in probing the coordination, quantity, and location of aluminum atoms in chemically treated zeolites but less useful than %3i NMR for direct structural determination.
Introduction Zeolites are crystalline aluminosilicates composed of comer-sharing S i O S and NO4" tetrahedra. The negative charge of the aluminosilicate matrix is neutralized by a cation, typically sodium. Zeolite frameworks are open, incorporating networks of interconnecting channels and cavities of molecular dimensions. Apart from the exchangeable cations, uncharged species, typically water, can be admitted into these as guests. Aluminum can in principle be present in zeolites both as a tetrahedrally coordinated entity in the aluminosilicate framework and also in intracrystalline spaces, in a variety of ionic and uncharged species with either octahedral or tetrahedral coordination. The ordering of aluminum and silicon in zeolitic frameworks is of particular interest since, inter alia, it dictates catalytic activity. However, studies of such ordering are very difficult by conventional methods. The reasons for this are the very similar scattering powers of Si and Al for X rays and the difficulty of obtaining single crystals of sufficient size and perfection, particularly in the case of synthetic zeolites which are now in the majority. High-resolution (magic-angle spinning) solid-state %Si NMR (MAS NMR) spectroscopy was recently shown to ~ zsSi be of great value in tackling this p r ~ b l e m . ~ -The ~~
~
(1)Department of Chemistry, Brock University, St.Catharinee,Ontario, Canada. (2)E. Lippmaa, M. MAgi, A. Samoson, G. Engelhardt, and A.-R. Grimmer, J. Am. Chem. SOC.,102,4889 (1980). (3) (a) S. Ramdae, J. M. Thomas, J. IUinowski, C. A. Fyfe, and J. S. Hartman,Nature (London),292,228(1981);(b) J. IUinm?d,S.F&mdas, J. M. Thomas, C. A. Fvfe, Faradav - - and J. S. Hartman. J. Chem. SOC.. Tram. 2, in press. (4) J. Klinowski, J. M.Thomas, C. A. Fyfe, and J. S. Hartman,J. Phys. Chem., 85, 2590 (1981). 0022-3854182/2086-1247$07.25/0
NMR spectrum clearly and quantitatively resolves the populations of Si(4Al), Si(3Al),Si(2Al), Si(lAl), and Si(0Al) groups. (Si(nA1) denotes here a tetrahedron connected to nA104" tetrahedra, and (4 - n) other SiOttetrahedra.) =Si NMR reveals the detailed distribution of tetrahedral atoms in a number of zeolites with Si/Al ratios close to unit? and in a range of synthetic faujasites (zeolitesX and Y)where this ratio varied between 1.19 and 2.73.3 However, as an aluminosilicate becomes progressively siliceous, =Si NMR becomes less useful as a means of ascertaining the environment of A1 atoms. Moreover, =Si NMR gives no unequivocal clue as to the presence of tetrahedrally coordinated aluminum or octahedrally coordinated aluminum outside the framework. It is clear that 27AlNMR spectroscopy is potentially of value for structural studies of aluminosilicates. So far some information about aluminum-oxygen coordination in these systems came from studies of the A1 KO X-ray emission and the antisymmetric M-0 vibration is observed in infrared spectra.* The present work uses nAl magnetic resonance as a tool and is an extension of our earlier structural investigations of by means of '%i NMR. nAl is a quadrupolar nucleus with spin of 5/2. It is 100% abundant, with relative sensitivity with respect t o the proton of 0.21. Three types of interaction have therefore to be considered the quadrupole interaction between the nuclear quadrupole moment and the electric field gradient, (5)L. A. Bursill, E. A. Lodge, J. M.Thomas, and A. K. Cheetham, J . Phys. Chem., 85,2409 (1981). (6)E. W. White and G. V. Gibbs, Am. Mineral., 54, 931 (1969). (7)G: H.KCihl in "MolecularSieves",J. B. Uytterhoeven,Ed.,Leuven University Press, 1973,p 227. (8) P. Tarte, Spectrochim. Acta, Part A, 23, 2127 (1967).
0 1982 American Chemical Soclety
