Photon Energy Relaxatlon and Thermal Effects on Gas-Phase

J. Phys. Chem. 1981, 85, 241-245

241

Photon Energy Relaxatlon and Thermal Effects on Gas-Phase Electronically Excited Methyl Salicylate A. U. Acuiia,” J. CatalBn, and F. Torlblo Instnuto de Qdmica Fislca “Rocasokno”,C.S.I.C., Madrid-6, Spain, and Depaflamento de Quimca Fisica y Cudntica, Unlversidad Autanoma de Madrid, Spain (Received: August 15, 1080)

The dual emission from gas-phase methyl salicylate is analyzed, invoking the previous model of two ground-state rotamen with intramolecular hydrogen bonds. It was found that the 450-nmfluorescence yield in the collision-free limit depends on the excess energy of the excited molecule, while in cyclohexane solution the quantum yield (0.022 f 0.002) is wavelength independent. We present evidence here consistent with the existence of a fast nonradiative route for deactivation of the excited zwitterion at 700 f 200 cm-’ above the 0-0electronic transition. The variation in gas-phase emission at 330 and 450 nm with temperature was determined and compared with previous data. All these temperature results were explained in terms of an activated relaxation process with a small contribution from the shift in the ground-state equilibrium, in contradiction with the existing hypothesis. It is proposed that the wavelength and thermal effects are properties of a single highly efficient radiationless mechanism operating in gas and condensed phases. The nonradiative decay is enhanced by oxygen with almost unit collision efficiency. Introduction Although some of the complexities present in the emission of salicyl derivatives were already noticed in early fluorescence research,12it was much later that a theoretical framework was available to handle the peculiar photophysics of these molecules. Thus, Weller was the first to p r ~ p o s ean ~ ?intramolecular ~ proton transfer reaction in the excited state to account for the strong red shift (ca. 10000 cm-’) of the blue (450 nm) emission (FB) in the dual fluorescence of the methyl salicylate parent molecule. This early assignment had not been invalidated by the copious research effort in this area,6 stimulated by both the theoretical interest of the phenomenon of dual emission and the industrial use of a number of ortho hydroxy aromatic compounds as UV stabili~ers.~ On the other hand, there was no such agreement on the origin of the near-UV (330 nm) component (FU) in the emission of methyl salicylate (MS). Weller proposed4that an equilibrium between two excited-state tautomers was established after the absorption of exciting light by a single ground-state species. He claimed support for this hypothesis from the similar quenching efficiency by CS2for the FB and FU. Besides, he estimatedlOJ1from the temperature dependence of the ratio of both emissions a proton transfer equilibrium enthalpy of -0.7 kcal/mol, with an upper limit of 0.12 kcal/mol for the activation energy of the excited-state transfer reaction. The idea of an excited-state equilibrium has been now abandoned, because it was found that the FB and FU in solution have different excitation spectral2and lifetimes.8 Different ground-state species were then proposed to explain this and the variation in the emission spectra of MS with solution polarity.13J4 We claimed recently15that a (1) Ley, H.; Engelhardt, K. V. 2.Phys. Chem. 1910, 74, 1. (2) Marsh, J. K. M. J. Chem. SOC.1924, 125, 4180. (3) Weller, A. Naturwissenschaften 1955, 42, 175. (4) Weller, A. Z. Elektrochemia 1956, 60, 1144. (5) See, e.g., ref 6-8 for recent reviews. (6)Klopffer, W. Adu. Photochem. 1977, 10, 311. (7) Goodman, J.; Brus, L. E. J . Am. Chem. SOC.1978,100,7472. (8) Smith, K. K.; Kaufman, K. J. J. Phys. Chem. 1976,82, 2286. (9) Trozzolo, A. M. In “Polymer Stabilization”; Hawkins, W. L. Ed.; Wiley: New York, 1972. (10) Weller, A. Prog. React. Kinet. 1961, 1, 189. (11) Beens, H.; Grellmann, K. H.; Gurr, M.; Weller, A. H. Discuss. Faraday SOC.1965, 39, 183. (12) Klopffer, W.; Naundorf, G. J. Lumin. 1974,8, 457. (13) Kosower, E. M.; Dodiur, H. J. Lumin. 1975/76, 11, 249. 0022-3654/81/2085-0241$01,00/0

ground-state equilibrium of the type shown in Figure 1 with only two rotamers can explain the relative intensity of the two emissions from MS and related compounds in a variety of solvents. In principle, gas-phase studies of the photophysics of MS and similar derivatives should give information free from the additional complexities of solvent interactions on the intramolecular H bonds. In this regard, apart from a pioneering work2 on gas-phase MS, where multiple fluorescence was clearly detected, we are aware of only one research effort16describing a limited number of observations on the emission behavior of MS vapor diluted in nitrogen. In this work, the interesting question of thermal effects on the fluorescence intensity ratio was also explored. This is a potentially useful source of information for any equilibrium involved in the ground state. However, contradictory results have been published. Thus, Klopffer reports in his paper16that a temperature rise increases the FU intensity, whereas the FB remains temperature independent. This is just the opposite behavior found in a picosecond study8 of MS proton transfer in inert solvents, where a continuous decrease of the FB intensity and lifetime with increasing temperature was observed. A similar decrease for FB was again reported recently1’ but coupled to an increasing FU intensity. We here report the results of a study of the emission from gas-phase MS in the collision-free region. A further investigation of thermal effects on vapor MS was also carried out, trying to ascertain the influence of wall adsorption16and spectral contamination1’ on the conflicting results described above. Quantum yields in gas and condensed phases and their dependence on excitation energy and quenching gases were measured. These results served to illustrate the properties of a highly efficient radiationless channel that is probably responsible for the striking photostability of MS and related compounds. This relaxation process is in some aspects similar to the controversial channel 111 of benzene,18 although the detailed (14) Sandros, K. Acta Chem. Scand., Ser. A 1976,30, 761. , U.; Amat-Guerri, F.; Catalan, J.; Gonzilez-Tablas, F. (15) A c ~ a A. J . Phys. Chem. 1980,84,629. (16) Klopffer, W.; Kaufmann, G. J.Lumin. 1979,20,283. (17) Ford, D.; Thistlethwaite, P. J.; Woolfe, G. J. Chem. Phys. Lett. 1980, 69, 246. (18) Jacon, M.; Lardeaux, C.; Lbpez-Delgado, R.; Tramer, A. Chem. Phys. 1977,24, 145.

0 1981 American Chemical Society

242

The Journal of Physical Chemlstty, Vol. 85, No. 3, 1981

Acuna et ai.

CH3

I

IC

IIC

Flgure 1. Ground-state closed tautomers of methyl salicylate.

mechanism can be quite different.

Experimental Section Materials. Methyl salicylate (Merck) was purified by fractional vacuum distillation and analyzed by GC. Cylinder N2 (S.E.O., 99.99%) and O2(S.E.O., 99.96%) were passed through activated molecular sieves at 193 K. nButane (Matheson Co.) was condensed at 77 K, degassed, and fractionally distilled from 178 to 77 K. All solvents used were purified as before.ls Methods. The vapor of MS was handled in a grease-free high-vacuum line, where absorption and emission cells could be evacuated to lo+ torr. The pressure of added gases was measured with a Wallace and Tiernan gauge, while the amount of MS, usually less than 0.2 torr, was determined by absorption spectrophotometry using published16 extinction coefficients. Emission spectra were recorded on a photon-counting SLM spectrofluorimeter, with a double grating excitation monochromator, and transferred to a PDP 11/05 computer for subsequent analysis. Very narrow excitation slits and the better resolution of gas-phase spectra made possible the measurement of both FB and FU intensities with little overlapping. The absence of mixing of the two bands is essential in recording their temperature dependence. It was also demonstrated by control experiments that the detected emission proceeds from gas-phase MS and not from the compound adsorbed on the walls. The sample compartment on the fluorimeter, thermostated at f0.2 “C,was fitted with an analyzing light perpendicular to the exciting beam, the sample cell optical density being measured under the same emission conditions. All spectra presented here, in photon units, were corrected. Emission calibration factors were determined by using a halogen-tungsten filament lamp. The excitation setup was calibrated with rhodamine B as photon counter,Igafter modification of the original “reference channel” of the fluorimeter for adequate performance. Quantum yields were determined by using two reference substances to by-pass the uncertainty associated20with the n2 geometrical factor. Quinine bisulfate in 0.1 N H2S04 ( 4 =~ O.5Q2l was selected for liquid solutions, as its emission spectrum completely overlaps that of FB. In gas phase, benzene at 20 torr (& = 0.19)22was used. Our data show that the fluorescence efficiency of gas-phase MS relative to itself in solution is the same as the ratio of the two independent determinations of the quantum yields, if the n2 correction is not applied. The error in our gas-phase yields (30%) is determined by the significant contribution to the observed optical density from MS adsorbed on the 1-cm cell walls. On the other hand, relative quantum yields (19)Yguerabide, J. Rev. Sci. Instrum. 1968,39, 1048. (20) Morris, J. V.; Mahaney, M. A.; Hhuber, J. R. J. Phys. Chern. 1976,

80,969. (21) Velapoldi, R. A. J. Res. Natl. Bur. Stand. (U.S.), Sect. A 1972, 76,641. (22) Rockley, M. G. Chem. Phys. Lett. 1977,50, 427.

Figure 2. Dual emission from gas-phase methyl salicylate. Excitation (A) and fluorescence (8)spectra In colllsiokfree conditions (-), diluted with 200 torr of N, (- -) and with 200 torr of O2 (-). The exciting wavelengths in (8)were 300 and 322 nm.

-

2

t

-

1 4

L 0’

im

200 PO? /torr

Flgure 3. Stern-Volmer plots of the quenchlng by O2 of the dual fluorescence of methyl salicylate at p N 0.1 torr. The excltlng wavelengths were 300 nm and 325 nm.

as a function of excitation wavelength, computed from absorption spectra obtained in 10-cm cells, have much greater accuracy. A Cary 17 spectrophotometer with a temperature-controlled holder was used for that purpose.

Results Quenching and Quantum Yields. Excitation of MS at low pressure gives rise to the structureless dual flourescence shown in Figure 2, together with the excitation spectrum of FB and FU. Under these conditions the time interval between collisions is about three orders of magnitude higher than the emission lifetime (vide infra). The addition of a diluent gas, such as nitrogen or n-butane, does not measurably change the envelopes of the two emission bands, although there are important changes in the excitation spectrum, as we will show below. In the presence of oxygen both the FB and FU intensities are quenched, but to a different extent. The quenching of the emission excited at 300 and 325 nm and analyzed by using simple Stern-Volmer kinetics is presented in Figure 3. As the total pressure was not kept constant in these experiments, the possibility exists that the molecules being quenched were thermalized by collisions to a different extent, depending on the quencher pressure. This would be more likely when the quenching efficiency is low. In Figure 4 the effect of collisional equilibration on the excitation spectrum of FB is presented. Together with the collision-free spectrum was included a normalized spectra in the presence of a moderate pressure of n-butane and in cyclohexane liquid solution, where it may be considered to have an internal pressure of about lo3 atm. In this last case the excitation spectrum closely follows the first absorption band, i.e., the relative quantum yield is wavelength independent, at least throughout this range. In the

The Journal of Physical Chemistry, Vol. 85,No. 3, 7987 243

Electronically Excited Methyl Salicylate B

t

I

I

350

,

I

h/nm

t l

1

I

350

h/nm

Figure 5. Variatlon in the emission spectra with temperature at low pressure for excitation at 300 nm (A) and at 327 nm (6). Flgure 4. The effect of colllsional relaxation on the 450-nm emisslon. (A) Normalizedexcltation spectra in collision-free conditions (-), with 600 torr of n-butane (- - e -), and in a 5 X lo4 M cyclohexane solution (---). The points (0)are from the absorption spectrum in the last solvent. (6)Wavelength dependence of the relative quantum yields (0)and absorption spectrum (- - -) at low pressure.

-

TABLE I: Photophysical Parameters of the 450-nm Emission from Methyl Salicylate at 295 K

@F TF/

PS

vapor" cyclohexane solidb*' 0.01 i 0.003 0.022 i 0.002 0.038 280' 460 120 i 20f

kne/ 8.2 X

lo9

3.7 X

lo9

solid 4 Kd If 1.2 x 104

2.1 x 109

Sa p = 0.1 torr; excitation at 327 nm. Dissolved in octadecane. ' From ref 8. Argon matrix; from ref 7. e k, = [ ( 1 / @ -~ 1 ])k R . f Estimated by assuming the radiative rate constant phase independent.

vapor phase, on the other hand, only a limited range of excitation energies gives rise to fluorescence,as can be seen in Figure 4B. Absolute values for the FB quantum yield are included in Table I. These data, and the relative values of Figure 4B, show that the gas-phase absolute quantum yield in the proximity of the 0-0 transition energy' approaches that of the cyclohexane solution. A value of 700 f 200 cm-' can be estimated for the threshold of the emission loss from the spectra of Figure 4, not far from kT at room temperature. The quantum yield of the FU component cannot be determined because the extent to which rotamer IIc absorbs is not known. The applicability of methods for measuring the emission efficiency without knowing the optical density of the emitterz3is, in this case, quite complex. Nevertheless, comparative experiments show that the ratio of FB to FU corrected intensities is about two times higher in cyclohexane than in the vapor phase, with exciting wavelengths at 300 and 327 nm, respectively. Thermal Effects. Measurements of the dual fluorescence of MS at low pressure as a function of temperature are presented in Figure 5. Our results are apparently similar in some aspects to the behavior described for a cyclohexane solution1' and in disagreement with Klopffer's experiments16in Nz-diluted MS vapor (vide supra). We think that the discrepancies stem from the fact that the pressure of MS involved in the present and previous work is close to the saturation pressure, and therefore physical adsorption on the fluorimeter cell walls might take place. (23) Britten,A.; Archer-Hall, J.; Lockwood, G.Analyst 1978,103,928.

1 ,P" " "

T/.C

6o

TPC

Flgure 6. Varhtion of the absorbance at 300 nm (A) in the fluorimeter cell with temperature. The thermal dependence (B) of the dual 0)and corrected (I 0) , emission at low pressure. Expermental(0, intensities with the data from part (A). Calculated curves (-) were obtained with the model described in the text.

Besides, the polarity of MS molecules renders its chemisorption on silica possible. The different temperature coefficients of the two processes, complicated with their diffusion dependence, will produce fluorescence changes large enough to overwhelm those of the photophysical rate constants. In an attempt to correct our meaurements for these factors we followed the changes in the gas-phase MS concentration as a function of temperature by measuring the optical density, either with the simultaneous recording of the emission or in separate experiments. The average values of various measurements are plotted in Figure 6A, while Figure 6B shows the corrected temperature dependence of the dual emission at low pressures. Corrected values were calculated by referring the observed intensities to a constant gas-phase concentration of MS. It is seen that these corrections are quite considerable for the FU component. The Franck-Condon emission envelopes and excitation spectra of the dual fluorescence did not show any measurable change in this temperature interval. On the other hand, we were not able to detect thermal or photochemical reaction products under these conditions. Discussion Emission Yields and Lifetimes. The only previous value for the FB quantum yield that we are aware of can be computed from the reported lifetimes8 of a methylcyclohexane solution, excited with a high-power laser pulse at 264 nm. This value (0.03) compares well with that for cyclohexane from Table I and is consistent with our as-

The Journal of Physical Chemistry, Vol. 85, No. 3, 1981

244

sertion about a wavelength-independentquantum yield in inert solvents. On the other hand, the FU quantum yield which cannot be measured directly (see Results) has to be much higher. Otherwise the absorption from FU emitters would have been detected as a distortion on the highfrequency side of the FB excitation spectrum of Figure 4 in cyclohexane. The gas-phase lifetimes for the FB can be computed from its quantum yield (Table I) and the lifetime in inert solvents8or, alternatively, from the radiative rate constant at 4 K7 if it does not change with the solvent electric field. The two ways predict a value of 120 f 20 ps for MS in the collision-free region excitated at 327 nm. It can be expected that this lifetime, as in other aromatic molecules,18 will show a similar excitation-energydependence with the quantum yield (Figure 4B). In fact, there are preliminary results from another laboratoryz4confirming this expectation. In an argon matrix at 4 K the lifetime of the FB excited at 323.1 nm is 12 n ~in ,good ~ agreement with the roomtemperature radiative lifetime computed from the quantum yield (Table I) and lifetimes in inert solvents. Therefore, the low temperature quantum yield should be unity. The lifetime of the FU component is not known accurately, although there is some agreement8J7pz4 on a value close to 1.2 ns in nonpolar solvents. With this value and the relative intensity of the two FB and FU emissions, both in the gas phase and in solution, it is possible to estimate a lifetime of -1.1 ns for the gas-phase FU component. The O2quenching experiments do not show the striking deviations from linearity (Figure 3) as those described before17for carbon tetrachloride. Thus, from the SternVolmer constant for the FB and the estimation of the lifetime at the exciting wavelength (120 ps) a quenching constant of KO N 1.3 X loll M-ls-l can be obtained. This is close to the Lard-sphere encounter rate. Hence, the FB is quenched every one or two collisions with O2molecules, as occurs in benzenez5emission. Our estimation for the quenching constant of the FU band, ko, N 0.6 X lo1' M-' s-l, indicates that only one collision in four is effective. This value would obviously be very sensitive to the emission lifetime. Temperature Effects on Relaxation Rates. The increase of the FU intensity with increasing temperature either in gas phase16 or in inert solvents17was interpreted before as being due to a shift of ground-state equilibria between different isomers. However, as shown above, when MS desorption from the cell walls is taken into account the intensity rise in gas phase is abolished. We feel that in solution also any change in the ground-state concentration of the emitting species should be very small. This can be deduced, for example, from the changes previously reportedS,l7for the FB in cyclohexane. Comparing both sets of results one can show that the FB intensity and lifetime temperature coefficients are almost the same, 5 and 4-5 kcal mol-', respectively. Obviously, an important change in the concentrtion of absorbing molecules with temperature would be reflected in the first parameter and not in the second one. Thus, we conclude that thermal effects on the dual fluorescence of MS in the range studied here are mainly the consequence of changes in the rates of radiationless processes. According to that, a nonlinear least-squaresfitting of the temperature dependence of both components was carried out, and is shown in Figure 6. The ~

~~~~~~

(24) Lbpez-Delgado, R.; Lazare, S. Personal communication. (25) Cundall, R. B.; Ogilvie, S. Mc. D. In "Organic Molecular Photophysics"; Birks, J. B., Ed.; Wiley: New York, 1975; Vol. 2, p 42.

Acuna et al.

number of input parameters (rate and equilibrium constants) was kept to a minimum compatible with realistic quantum yields and lifetimes. The discrepancy between experimental and computed data could of course be made negligible by increasing this number. It can be seen that for the FB a simple model based on a temperature-dependent deactivation channel could account for most of the temperature effect. If we assume that the radiative lifetime is 12 ns, the experimental quantum yield is reproduced if the radiationless rate constant has an activation energy of 5 kcal mol-l, with a frequency factor of 4 X 1013s-l. It would be interesting to study these thermal effects at different excitation wavelengths, but the small pressure of MS and the rapid thermal quenching with moderate temperature increases makes the analysis uncertain. The lack of absolute quantum yields and accurate lifetimes for the FU component precluded the establishment of a unique set of fitting parameters. Here we made the reasonable assumptions that the gas-phase FU lifetime is 1f 0.2 ns (vide supra) and that the extinction coefficients of the two rotamers in Figure 1 differ by less than 15% at 300 nm. On that basis, the thermal dependence of this emission (Figure 6B) can be approximated by a temperature-dependent radiationless rate (k: = 2 X 10lz exp(5000/RT) s-') together with a small change in the absolute concentration of the emitting species. This change may be attributed to a shift in the ground-state equilibrium (Figure l),with AHe = -4 kcal mol-l. If we ignore the entropy difference between rotamers ICand IIc, an increase in the concentration of the last from 0.2 to 0.6% over our temperature range can be computed (Kern= 0.001). Thus, the simultaneous change in the concentration of FB emitters would be undetected under the present experimental conditions. These parameters predict finally a quantum yield of 0.27 for the molecules emitting the FU. Conclusions The thermal quenching of the fluorescence at 450 nm from methyl salicylate either in gas phase or in solution in inert solvents is not the consequence of a shift in a ground-state equilibrium between stable rotamers. Rather, the observations described here and before are consistent with the presence of an efficient radiationless pathway connected with molecules of ICgeometry (Figure 1).This decay channel should be energetically very close to the absorption band 0-0transition, perhaps by about 700 cm-' or even less. When methyl salicylate is collisionally relaxed, as in liquid solutions, the wavelength dependence of this dissipative channel is absent. However, it can still be detected by thermal activation of the absorbing molecules. Since temperature e f f e h in liquid solution8are very similar to gas-phase results described here it is possible that the same relaxation process is operating in both phases. There are some differences between the thermal activation energy and the optical threshold. This should be expected, because the thermal deposition of energy in the decay-promoting mode(s) must follow a Boltzmann distribution among all molecular degrees of freedom. Optical excitation, on the other hand, can be highly selective. If our estimation of the energy threshold for the radiationless channel is correct some of the gas-phase photophysical data presented in this work are then complex average values. This is a consequence of the dispersion of emission probabilities in the collision-freeregion, even at room temperature. We think that the experiments described here may provide further insights into the photophysics of methyl salicylate that could be generalized to some of the o-

J. Phys. Chem. 1981, 85, 245-248

hydroxy derivatives showing excited-state proton transfer. However, the nature of the elusive6p8-26 mechanism of this fast energy-dissipating process remains unknown. We are exploring the possibility that this “funnel” lies in the configurational space to be travelled by the Franck-Con(26)Lamola, A. A.; Sharp, L. J. J. Phys. Chem. 1966,70, 2634.

245

don excited molecules before being transformed into blue light emitters. -

Acknowledgment. We express our gratitude to Dr. J. Gonzdez for laboratory facilities and to Dr.C. Gutierrez for his help in many different ways. This work was supported byproject IIIP-$O42 of the C.C.H.N. and by the Comisi6n Asesora Cientifica y Tecnica.

Hcckel Theory Examination of the Hydrodenltrogenatlon of Pyrldlne Anthony J. Duben Chemistry Department, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701 (Received: September 2, 1980)

In order to examine the relative ease by which the cataytic hydrodesulfurizationof thiophene and the catalytic denitrogenation of pyridine proceed, I have used simple Huckel molecular orbital theory to study both reactions. In both cases, the description of the catalyst (presumed to have been a typical cobalt-molybdenum catalyst) was kept the same in order to achieve consistency in the treatment of the two problems and comparability of the results. Application of the model yielded results consistent with the experimental observation that denitrogenation is more difficult than desulfurization. Six-membered rings, such as pyridine, produced a different distribution of ?r molecular orbitals than were obtained with a five-membered ring, such as thiophene, in both the free and adsorbed states. Ring connectivityappeared to have been the primary factor affecting the electronic structures of the species undergoing the catalytic reactions.

Introduction The hydrodenitrogenation of pyridine and pyridine-like gecieg in the presence of a heterogeneous catalyst is an important part of the processing of kerogen-derived liquids from oil shale. In the processing of petroleum, removal of the nitrogen occurs at the same time that sulfur is removed from the thiophenes and mercaptans in the feedstocks. Removal of nitrogen from petroleum usually occurs without much difficulty since nitrogen-containing compounds are in rather low concentrations compared to the concentrations of sulfur compounds. In oil shale derived liquids, nitrogenous heterocyclics (many of which are derivatives of pyridine) are the predominant heterocyclic contaminants. Because of the concentrations in which they are found, more severe conditions are required for their removal than is needed for processing thiophenes.’I2 Much of the present technology for hydrodenitrogenation is simply that for hydrodesulfurization although the suitability of that technology can be questioned because of the severity of the conditions used. Experimental work indicates differences between the two reactions. Specifically noteworthy are the apparent necessity of hydrogenating the pyridine ring prior to breaking the carbon-heteratom bonds3 and the presence of disproportionation reactions amog the hydrogenated nitrogen h e t e r o ~ y c l e s . ~ Kinetics ~~ experiments indicate no significant difference between the rate of denitrogen(1) R. A. Flinn, 0. A. Larson, and H. Beuther, Hydrocarbon Process. Pet. Refiner, 42,129 (1963). (2)C . N.Satterfield, M. Modell, and J. F. Maver, - . AIChE J., . 21,1100 . (1975). (3)A. K.Aboul-Gheit, Can. J. Chem., 53, 2575 (1975). (4)J. Sonnemans and P. Mars, J. Cutal., 34, 215 (1974). (5)J. Sonnemans, W.J. Neyens, and P. Mars, J.Catal., 34,230(1974). 0022-365418112085-0245$01.OO/O

TABLE I: Huckel Parameters Used .(X) = 01 t hxP P(XY) = knvB ~~

atom N C

metal d,. metal dsy

hX 0.5 (pyridine) 2.0 (pyridinium) 0.05 (pyridine) 0.20 (pyridinium) -1.25 -1.35

kXY CN = 1.0

d,,N d,,N

= 0.25 = r0.10

ation of piperidine and pyridine.6 Apparently, hydrogenation of the ring is much faster than C-N bond scission. The major problem appears to be that of explaining why pyridine must become fully hydrogenated before bond breaking in contrast to the behavior of thiophene. Interpretations based on the molecular electronic structure of the chemical species involved in the reaction could lead to an understanding of why problems arise in the application of hydrodesulfurization technology to the hydrodenitrogenation process. Basic Model In previously published work on the desulfurization of thiophene,I the Lipsch and Schuit model was examined by using simple Huckel molecular orbital theory. In this paper, Huckel molecular orbital parameters were obtained for the metal d,,(bl) and d,,(az) orbitals. The same parameters for the metal are retained in this study since the reactions of thiophene and pyridine are assumed to occur at the same metal site in order to make comparisons between the two reactions. The symmetry group descriptions (6)H.G. McIlvried, 2nd. Eng. Chem. Process Des. Deu., 10,125(1971). (7)A. J. Duben, J . Phys. Chem., 82 348 (1978).

0 1981 American Chemical Society