J. Phys. Chem. 1988, 92, 5926-5930
5926
Gas-Phase Protonation of Hexamethylphosphoric Triamide Analogues XP(NMe,),, X = 0, S, Se. A Theoretical and Experimental Study Jacques Weber* Laboratory of Computational Chemistry, University of Geneva, 30 quai Ernest Ansermet, 121 I Geneva 4, Switzerland
and Raymond Houriet Institute of Physical Chemistry, EPFL-Ecublens, 1015 Lausanne, Switzerland (Received: March I , 1988)
Gas-phase basicities were determined in an ion cyclotron resonance spectrometer for the XP(NMe2)3,X = 0, S, Se, series of compounds, leading to the ordering 0 > S > Se for the stability of the protonated species. Ab initio self-consistent field (SCF) calculations using a large, flexible basis set and performed on the XP(NH2), model compounds conclude the same ordering as to proton affinities, the site of protonation being located on chalcogen atom X, and provide the basis for a general discussion of the mechanism of protonation. As an example, considerable differences in the PXH' bond angle are observed in the protonated compounds, both at the molecular electrostatic potential and SCF levels. This is ascribed to the different nature of the X-H+ bonding, as demonstrated by a decomposition of the HC--XP(NH2)3interaction energies according to the procedure suggested by Morokuma. Finally, zero-point vibrational and correlation corrections are estimated by calculations performed on OPH3 and SPH3,which allows us to conclude that the ordering obtained at the SCF level for the proton affinities of the XP(NH2)3series should not be modified by these corrections.
Introduction In a recent study,I we investigated the protonation of phosphoramides OP(NR2), using both ion cyclotron resonance (ICR) data and ab initio S C F calculations, concluding that the protonation site is most probably located on oxygen rather than on nitrogen, which leads to substituted phosphonium ions HO-P(NR2)3+. These deductions were supported by both the effects of substituents R on gas-phase basicity (GB) of OP(RN2), and by the changes in geometries between the neutral and protonated forms of these compounds as indicated by the molecular orbital (MO) results. Furthermore, an energy difference of about 100 kcal/mol favoring 0- versus N-protonation was obtained in calculations performed for the OP(Me)2NMe2model compound. To our knowledge, the problem of the stabilization of a typical phosphonium ion HX-P(NR2)3+ brought about by differing adjacent heteroatoms X has not been investigated. Consequently, we have used in the present study the same dual approach, i.e., theoretical and experimental, to elucidate the basicity properties of the analogues of hexamethylphosphoric triamide (HMPT), XP(NMe2)3, with X = 0, S , Se (1-3). Our purpose is to investigate in detail the mechanism of protonation of H M P T and its thio and seleno analogues in terms of energetics and structural features and also to determine the trend in gas-phase proton affinities (PA) within the 1-2-3 series, to compare with the ordering 0 > S > Se recently observed for chalcogen-substituted ethylenes2 and for which protonation occurs on the &carbon. In addition, a decomposition of the calculated PA values in terms of their main components (electrostatic, charge transfer, polarization) is performed3 to shed more light on the very nature of the interaction between chalcogen (X = 0, S, Se) in XP(NMe2)3 and the incoming proton.
Computational Details The calculations were performed on XP(NH2)3 model compounds, X = 0,S, Se, which are the simplest triamidophosphine chalcogenides and represent undoubtedly good prototypes of 1, 2, and 3. All these ab initio calculations were carried out at the ( I ) Bollinger, J. C.; Houriet, R.; Kern, C. W.; Perret, D.; Weber, J.; Yvernault, T. J . Am. Chem. Soc. 1985, 107, 5352. (2) Osapay, K.; Dehalle, J.; Nsunda, K. M.; Rolli, E.; Houriet, R.; Hevesi, L. Experimental and Theoretical Studies of the Gas Phase Protonation of Vinyl Ethers, Vinyl Sulfides and Vinyl Selenides; to be submitted for publication. (3) Morokuma, K. Acc. Chem. Res. 1977, 10, 294.
S C F level on a VAX 8700 computer using standard versions of GAUSSIAN go4 and 8zS programs. Gradient optimization techn i q u e ~ ~have ~ ' been employed to optimize fully the geometries of these bases and their corresponding protonated species. The double-( contracted Gaussian basis sets proposed by Dunning8-lo have been used, augmented by a single d polarization function on phosphorus ( a = 0.5545), sulfur ( a = 0.6500),and selenium ( a = 0.4800), these exponents having been optimized for the neutral compounds. For evaluation of the electron correlation (A&) and zero-point vibrational (A&) energy corrections to proton affinities of the parent compounds OPH3and SPH3, additional MP4(SDTQ) and vibrational frequencies calculations have been performed for these latter species and their protonated homologues as well, using the same basis set as for the XP(NH2)3series. However, in this case again, the structures were optimized at the S C F level.
Experimental Details Gas-phase basicities were determined in an ICR spectrometer under conditions similar to those previously described,I-" from the equilibrium constant for the proton-transfer reactions between the phosphoramide analogues M and the reference bases B (eq 1): MH' + B s M + BH+ (1) GB(M) = GB(B) + AGr is obtained from the measurement of the equilibrium constant for reaction 1 (K,) and from the relationship AG, = -RT In K,, with T = 313 K. At least three independent measurements were carried out for each couple M/B with the pressure ratio varying in about a 5-fold range and total pressure of about (2-3) X lo4 Torr. The concentrations of the neutrals were determined from the pressure measurements of an ionization gauge. The correction for the gauge readings was (4) Binkley, R. A.; Whiteside, R. A.; Krishnan, R.; Seeger, R.; DeFrees, D. J.; Schlegel, H . B.; Topiol, S.; Kahn, L. R.; Pople, J. A. QCPE 1981, 13, 406. (5) Binkley, J. S.; Frisch, M. J.; DeFrees, D. J.; Raghavachari, R. A,; Whiteside, R. A.; Schlegel, H. B.; Fluder, E. M.; Pople, J. A. GAUSSIAN 82; Carnegie-Mellon University: Pittsburgh, PA, 1983. (6) Binkley, J. S. J . Chem. Phys. 1976, 64, 5142. (7) Fletcher, R.; Powell, M. J. D. Comput. J. 1973, 6, 163. (8) Dunning, T. H. J . Chem. Phys. 1970, 53, 2823. (9) Dunning, T. H.; Hay, J. P.Modern Theoretical Chemistry. Methods of Electronic Structure Theory; Schaefer, H. F., Ed.; Plenum: New York, 1977; Vol. 111, p 1. (IO) Dunning, T. H . J . Chem. Phys. 1977, 66, 1382. ( 1 1) Bollinger, J. C.; Houriet, R.; Yvernault, T. Phosphorus Sulfur 1984, 19, 379.
0022-3654/88/2092-5926$01.50/00 1988 American Chemical Society
Gas-Phase Protonation of XP(NMe2)3, X = 0, S, Se
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 5921
TABLE I: Calculated Structures of XP(NH2)3 Compounds and Their Protonated Homologues"
x=o r(PX) r(PN) r(")
r(XH+) f(XPN) L(PNH) L(PXH+) L(XPNH1) L(XPNH2) f (NPXH')
x=s
X = Se
neutral
protonated
neutral
protonated
neutral
protonated
1.47 1 1.650 0.996
1.548 1.611 1.ooo 0.952 108.7 122.5 136.3 -45.2 132.8 70.2
1.941 1.654 0.996
2.062 1.619 1.000 1.327 109.5 122.5 94.7 -45.6 132.7 55.3
2.099 1.654 0.996
2.210 1.622 1.000 1.453 109.7 122.5 91.7 -45.6 134.8 52.8
114.0 121.4 -33.3 133.1
114.6 121.1 -31.5 133.5
114.4 121.0 -30.8 133.9
"Distances are in angstroms, angles in degrees; the X P N H , and H P N H z dihedral angles have been assumed to be the same in all the NH2groups, and both have been optimized separately in each case. TABLE 11: Mulliken Gross Orbital Populations and Charges of XP(NH& Compounds and Their Protonated Homologues"
x=o X
ns nPU
nP* np(tota1)
x=s
protonated
neutral
protonated
neutral
protonated
1.826 1.390 3.468 4.858
1.776 1.343 3.627 4.970
-0.684 0.9 17 0.652 1.346 1.998 0.592 1.552
-0.746 0.905 0.593 1.322 1.915 0.566 1.673 0.505
1.949 0.992 3.505 4.497 0.042 -0.474 1.114 0.822 1.464 2.286 0.460 1.201
1.924 1.069 3.046 4.115 0.072 -0.088 1.126 0.809 1.416 2.225 0.471 1.239 0.188
1.996 0.876 3.586 4.462 0.100 -0.462 1.167 0.872 1.435 2.307 0.430 1.156
1.963 0.986 2.998 3.984 0.134 +0.028 1.175 0.852 1.400 2.252 0.450 1.184 0.135
d
gross charge P
3s 3PU
3PT
3p(total) 3d H+
gross charge gross charge
X = Se
neutral
'ns, npu, n p r , and nd correspond to the population of outer s, p, and d shells of X. The
estimated from the polarizability of the neutrals." Compound 2 was kindly provided by Prof. G. Gritzner (University of Linz, Austria) and compound 3 by Prof. J. Songstad (University of Bergen, Norway).
Results and Discussion Ab Initio Structures and Basicities. The optimized structures of the neutral and protonated species are presented in Table I. In agreement with both theoretical and experimental studies of OPX3 phosphoryl compounds,' protonation of the XP(NH2), systems occurs at the X chalcogen site. As a result of this Xprotonation, Table I shows that the PX bond length increases significantly. We have discussed in a previous paper1 the nature of the PO bond in such phosphoryl compounds. It therefore suffices to recall here that bonding between P and 0 may be described as a single a-bond, arising from the u-donation of the phosphorus lone pair to a vacant p orbital on oxygen, enhanced by some degree of a-back-bonding from the oxygen lone pairs to the low-lying vacant d orbitals of p h o s p h ~ r u s . ' ~ *The ' ~ increase of PX bond length upon protonation is thus undoubtedly due to a smaller back-bonding donation because one lone pair of the X atom is involved in bonding with the proton, which indicates that X-protonation reinforces the single cr-bond character of the PX bond. Similarly, the decrease in P N bond distances upon protonation originates presumably from the larger positive charge on phosphorus in the protonated species, which in turn leads to a larger back-donation from the N H 2 moieties and hence to smaller P N bond lengths. Simultaneously, the XPN bond angles decrease somewhat upon protonation so as to become nearly tetrahedral. Table I1 presents the results of a Mulliken population analysis performed for both neutral and protonated species. It is seen that, among the three compounds, OP(NH2)3is undoubtedly the one that exhibits the largest polar character of the PX bond. In (12) Kutzelnigg, W. Angew. Chem., Int. Ed. Engl. 1984, 23, 212. (13) Schmidt, M. W.; Yabushita, S . ; Gordon, M. S . J. Phys. Chem. 1984, 88, 382.
u
and
T
labels refer to the X-P bond axis.
addition, contrasting with the sulfur and selenium analogues, protonation of OP(NH2), reinforces the polar character of the PO bond, as shown in Table I1 where the gross charges on these atoms are larger in the protonated species than in the neutral compound. This may be interpreted as an indication of the predominant electrostatic character of the interaction between the oxygen atom in OP(NH2)3 and the incoming proton: the effects of charge redistribution within the OP(NH& molecule upon protonation are particularly important, the amino groups behaving as donors to the OH+ end, with the net result that they are responsible for the charge transferred to the proton and that the negative charge on oxygen increases due to its high electron-withdrawing character. In the sulfur and selenium compounds, however, the interaction between chalcogen atoms and the incoming proton is much more of a charge-transfer nature, as Table I1 reveals an important decrease of the negative charge on these atoms upon protonation, to the extent that the selenium atom in HSeP(NH2)3+bears a positive charge. One may therefore expect that the XH' bond is much more covalent in the sulfur and selenium compounds than in the oxygen one, which is confirmed by the 0 >> S > Se ordering of H+ gross charges after protonation (Table 11). The sharp differences between O H + and S H + or SeH+ bond characters are also reflected in the values of PXH+ bond angles reported in Table I. It is seen here that this bond angle reduces drastically from 136.3' in the oxygen compound to 94.7' in the sulfur one and to 91.7' in the selenium one. Clearly, the changes in this bond angle must correlate with the changing character of the XH' bond along the series of chalcogen atoms: in HOP(NH2)3+,due to the predominantly electrostatic character of the XH+ bond, the protonation site is strongly dependent upon the multipole moments of the charge distribution of the substrate molecule. As OP(NH2)3exhibits a large dipole moment oriented along the PO bond, the proton tends much more to align along this axis than in the sulfur and selenium compounds, where the charge-transfer interactions predominate. In the latter cases, the protonation site is expected to come closer to the directions of the lone pair orbitals of the X-donor atom, Le., perpendicular to the
5928
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988
Weber and Houriet TABLE 111: Calculated Proton Affinities of XP(NH& Compounds“ x=o x=s X = Se E(M) -582.508 53 -905.141 40 -2905.016 10 E(MH+) -582.878 96 -905.494 15 -2905.367 51
PA, exptlb PA, calcd Edcf
i
E, EPI
227.5 232.4 -12.4 102.5 61.3 81.0
223.8 221.4 -11.3 40.6 68.0 124.1
222.3 220.5 -9.7 37.9 69.4 122.9
E,, Total energies are in hartrees, proton affinities and energy components in kcal/mol. bMeasured for XP(NMe2),; values deduced from the experimental GB data of Table V by taking into account only the translational effect of the proton on the entropy at 313 K: PA = GB + 8.2, in kcal/mol. along the PS axis in SP(NH2)’. To perform an in-depth study of the protonation of the XP(”2)’ species, we decided to analyze the calculated proton affinities according to the energy decomposition procedure suggested by Morokuma.’ To this end, the proton affinity of molecule M is written as PA(M) = E(M)
Figure 1. Contour map of the MEP of OP(NH2), calculated in an OPN plane. Contour values of 0, 1, 2, 3, and 4 correspond to energies of 0, 20, 50, 80, and 99 kcal/mol, respectively. Positive (negative) contours
are indicated by a solid (dashed) line.
- E(MH+)
(2)
where E(M) and E(MH+) are total energies calculated for the optimized structures of M and protonated M, respectively. Defining E’(M) as the total energy of M calculated at the geometry of M in MH+, one has PA(M) = E(M) - E’(M)
=
Edef
+ E’(M)
- E(MH+)
+ Eint
(3)
(4)
where Edef,the structural deformation energy, is defined as Edef
= E(M)
- E’(M)
(5)
whereas Eint,the frozen geometry interaction energy of M with the incoming proton, is given by Eint= E’(M) - E(MH+)
(6)
According to the Morokuma component analysis,’ Eintmay be decomposed in our case as the sum of electrostatic (E-), polarization (Ep,), and charge-transfer (Ect) energies,I4 which leads to the following components of PA(M): PA(M) = Edef +
Figure 2. Contour map of the MEP of SP(NH2), calculated in an SPN plane. Contour values of 0, 1, 2, 3, and 4 correspond to energies of 0, 15, 22, 30, and 40 kcal/mol, respectively. Positive (negative) contours are indicated by a solid (dashed) line.
XP bond. However, we should mention that the same trend POH’
>> PSH+ in bond angles is displayed by the molecular electrostatic potential (MEPs) of XP(NH2),, X = 0, S, (Figures 1 and 2). Indeed, whereas the minimum of the MEP of OP(NH2), is located along the PO bond axis, revealing thus a collinear approach for the incoming proton, that of SP(NH2)’ is markedly out of the PS bond axis, the direction of the minimum from the S atom making an angle of about 1loo with the PS axis. Thus, at the electrostatic level only, the directions of approach of the incoming proton are significantly different, which may be easily understood on the basis of the Mulliken populations of chalcogen atoms (Table 11): whereas the p~ populations are roughly the same for both X = 0 and X = S (3.5e),the pu population reduces drastically from 1.39e in the oxygen to 0.99e in the sulfur compound, which explains the much smaller attraction experienced by the proton
+ Epl + E c t
(7)
Table I11 presents the calculated proton affinities together with their decomposition into main components. It is seen that the ordering PA(X=O) > PA(X=S) > PA(X=Se), deduced experimentally from measurements preformed on XP(NMe2)’ species, is well reproduced by our calculations, though performed at the S C F level, Le., without taking into account zero-point vibrational and electron correlation corrections. These corrections are expected to decrease the PA values obtained at the S C F level, as AEzpcorrections are in any case negative (presumably of the order of -5 to -10 kcal/mol for our compounds’), whereas AE, corrections, which may be of both signs,I5 have been shown to also decrease PA values in the case of OPX3 compounds.’ To evaluate the importance of both effects on oxygen versus sulfur protonation mechanisms, we have performed additional calculations on OPH, and SPH, (vide infra). These calculations should also help understand why the calculated PA of OP(NH2)’ is predicted to be larger than the experimental value, whereas for the sulfur and selenium compounds the calculated PAS are slightly smaller than their experimental counterparts. An interesting feature that emerges from Table I11 lies in the sharp differences observed in the PA components of the oxygen versus sulfur and selenium species. Whereas E,, is by far the (14) Kollman, P.; Rothenberg, S. J . Am. Chem. SOC.1977, 99, 1333. (15) Frisch, M. J.; Del Bene, J.; Raghavachari, K.; Pople, J. A. Chem. Phys. Letf. 1981, 83, 240.
Gas-Phase Protonation of XP(NMe2),, X = 0, S, Se TABLE IV: Calculated Proton Affinities of OPH3 and SPHB" OPHq SPHl -411.306 31 -739.951 44 E(M) -417.64422 -140.211 55 E(MH+) PA(SCF) 212.0 200.9 AEzg -6.9 -5.1 -10.3 -0.2 AEC PA(corr) 194.8 195.0
" Total energies in hartrees, proton affinities
and corrections in
kcal/mol. bCalculated at the MP4(SDTQ) level. largest component in the proton-molecule interaction in HOP(NH2),+, a much different trend is observed for the other two species where E,, predominates. This leads us to unambiguously classify the protonated oxygen compound as an "electrostatic ~ o m p l e x " characteristic ,~ of hard acid-hard base interaction according to the concepts proposed by Pearson.16 On the other hand, the interaction between proton and sulfur or selenium species is much more covalent, which allows us to describe the corresponding protonated compounds as "charge-transfer complexes", exhibiting a more pronounced soft acidsoft base character, which underlines the ambivalent character of the proton in both types of complexes. Clearly, these calculations are consistent with the results we have presented in Tables I and 11, particularly with the decreasing polarity of the X P bond along the series 0, S, Se and the simultaneous decrease of the positive charge on the incoming proton (Table 11). It is also seen in Table 111 that polarization energies increase slowly when going from the oxygen to the selenium compound, which correlates well with the increasing size of electron density around chalcogen atom. Table IV presents the results of the calculations performed on OPH, and SPH,. Compared with the results obtained for OP(NH,), and SP(NH2),, it is immediately seen that the PA values in these compounds are significantly affected in the presence of amino groups, which stabilize to a rather large extent the protonated forms through polarization effects. However, it is interesting to notice that the difference between calculated PA values of oxygen and sulfur species remains practically the same (1 1 kcal/mol) in both XPH, and XP(NH2), series of compounds. Examination of the 4Eq and AE, corrections reveals that whereas the former ones are close to each other for OPH, and SP,, the latter corrections are very different, as AE,is -10.3 kcal/mol for OPH, and only -0.2 kcal/mol for SPH,. As we use rather flexible one-electron basis sets of similar quality for both systems, this result is presumably not due to some calculation artifact or to the MP4(SDTQ) procedure itself. Analysis of the calculated energies shows that the origin of this difference in 4E, is as follows. Whereas correlation effects lower the S C F total energies of both SPH, and HSPH,' by the same amount, this lowering is about 10 kcal/mol smaller for HOPH, than for OPH,. In our opinion, this must be related to the large structural differences observed for the protonated species, since the calculated POH' and PSH' bond angles are 142' and 9 5 O , respectively. Recent MP4(SDTQ) calculations performed on protonated acetylene" have indeed shown that correlation effects are significantly larger (about 5 kcal/mol) for the nonclassical (bridged) structure, where the proton binds simultaneously to two carbon atoms, than for the classical one with the proton bonded to only one carbon atom (LCCH' = 180'). As the structure of protonated SPH, is reminiscent of a bridged single SP bond, whereas that of protonated OPH, is analogous to a classical H+-OP bond, correlation effects are expected to play a less important role in stabilizing the latter species, which explains the 10 kcal/mol difference in correlation corrections between HOPH3' and HSPH,'. As the structures of protonated XP(NH2), are very similar to those of protonated XPH,, it is reasonable to assume that the difference of 10 kcal/mol predicted for 4Ec between 0 and S compounds may be in large part reported on the triamido species, which would lead to PA(SCF) + AE, values lying much closer (16) Pearson, R.G.J . Am. Chem. SOC.1963.85, 3533. (17) Pople, J. A. Chem. Phys. Lett. 1987, 137, 10.
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 5929 TABLE V Experimental GB Values (kcal/mol) M B GB(B)' OP(NMe2)3 SP(NMe2), 4-picoline 215.9 3-picoline 215.9 Se(NMe2), triamylamine 213.9 cyclohexylamine 212.9
G,b
-0.25 -0.3 0.1 1.3
GB(M)' 219.3* 215.6 215.6 214.1 214.1
*
From ref 18 and 19. AGr is the free energy change for reaction 1. f 0 . 2 kcal/mol (deviation on the experimental determination of AGr). dFrom ref 1. one another for the 0 and S compounds, with the ordering PA(X=O) > PA(X=S) probably preserved. Actually, as shown by our calculations on XPH,, the only important difference in energy corrections to the PA values calculated at the S C F level for the XP(NH2), series should be found in 4E,, which suggests that both AEzpand the influence of substituting NMez by N H 2 groups should be of similar importance for X = 0, S, Se. This explains the good agreement between theory and experiment as to the ordering PA(X=O) > PA(X=S) > PA(X=Se). Our conclusion is therefore that the present calculations, even if performed at the S C F level, lead actually to a very satisfactory description of the protonation of chalcogen derivatives of phosphoramides and of the characteristics of the proton-chalcogen bonding as well. Experimental Basicities. The experimental GB values for the three analogues 1-3 are reported in Table V. HMPT is seen to be more basic than its thio analogue by 3.7 kcal/mol, and the latter more basic than selenio-HMPT by 1.5 kcal/mol. This ordering of basicities is clearly in contrast with the higher GB found for thio relative to the oxo analogues for systems in which the protonation results in the charge being localized on the heteroatom, Le., in alcohols, thiols, and t h i o e t h e r ~ . ' ~ This ~ ' ~ reinforces the conclusion that protonation of 2 and 3 occurs on the chalcogen site to form substituted phosphonium ions as already shown for HMPT.'*" To understand the changes in GB on going from 1 to 3, we shall consider that the overall effect of the dimethylamino substituents remains constant for the protonation processes 1 1H+, 2 2H+, and 3 3H+. The GB differences are thus interpreted as directly representing the influence of 0,S, and Se on the stability of the phosphonium ions 1H+, 2H+, and 3H'. The existing literature on the influence of the oxo, thio, and possibly selenio substituents on the stabilization of adjacent charge is very sparse and controversial, so that no trend actually emerges from these studies.20 We thus consider the influence of these substituents on the nature of the chemical bonding between P and 0 (or S or Se). As recalled in the preceding section, this bonding is based on the u-donation from the P lone pair to a vacant p orbital on 0 and simultaneously on some a-back-donation from the 0 lone pairs to the vacant d orbitals on P. Within this context, the major effect of protonation is to increase the PO bond length because one of the lone pairs is involved in bonding with the proton. As a consequence, protonation reduces the possibility for aback-bonding. If we interpret the present GB results along these lines, we may infer that the experimental ordering in GB (0 > S > Se) corresponds to a smaller reduction in *-back-bonding for 0 than for S and than for Se. This is undoubtedly confirmed by the calculations, which indicate a significantly smaller increase in PX bond length upon protonation for 0 than for S and Se (see Table I). Actually, it is not possible to substantiate any further these changes in the amount of a-back-bonding upon protonation by using the Mulliken population in the 3d orbital of phosphorus (Table 11), as an opposite trend is observed: the d population on P decreases upon protonation in OP(NH,),, whereas it increases in both SP(NH,), and SeP(NH,),, which seems to indicate a
-
-
-
(1 8) Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academic: New York, 1979; Chapter 9. (191 Lias. S . G.: Liebman. J. F.: Levin, R. D. J . Phys. Chem. Ref. Data 1984, i 3 , 695. ' (20) Bernardi, F.; Bottoni, A,; Venturini, A. J . Am. Chem. SOC.1986, 108, 5395,and references therein.
5930
J . Phys. Chem. 1988, 92, 5930-5933
larger reduction in r-back-bonding for 0 than for S or Se. However, the gross d population on phosphorus originates from the competition of many subtle effects, such as pd hybridization on this atom and back-bonding from both X and N H 2 groups, and the overall change in this population is not a good criterion for estimating the changes in back-bonding from chalcogen only. Using more simple and intuitive arguments, one might also argue that the proton, which behaves as a hard acid, prefers coordination with a hard base such as 1 rather than with soft bases such as 2 or 3, which explains the larger stability of 1H+. This is partly confirmed by the calculations, which demonstrate the predominantly electrostatic character of the 1H’ complex as compared with 2H+ and 3H+. In any case, the present study shows that
it is important to use a combined approach made of both ab initio calculations and ICR experiments so as to provide a comprehensive description of the mechanism of protonation of these HMPT analogues. Acknowledgment. This study has been supported by the Swiss National Science Foundation (Projects 2.806-0.85 and 2.673.0.85). We thank Professors G. Gritzner and J. Songstad for providing compounds 2 and 3. Registry No. 1, 680-31-9; 1 conjugate monoacid, 91516-76-6; 2, 3732-82-9;2 conjugate monoacid, 116149-05-4;3, 7422-73-3;3 conjugate monoacid, 116149-06-5; OPH,, 13840-40-9;OPHS.H*, 11614907-6; SPH3, 35280-73-0; SPHyH’, 116149-08-7.
Heavy-Atom-Induced Spin-Lattice Relaxation in the Photoexcited Triplet State of Naphthalene G. Kaiser and J. Friedrich* Physikalisches Institut and Bayreuther Institut fur Makromolekulforschung, Universitat Bayreuth, 0-8580Bayreuth, FRG (Received: March 9, 1988)
We have measured the average spin-lattice relaxation rate in the photoexcited triplet state of naphthalene dissolved in bromoethane-doped 3-methylpentane glass. Even at 1.5 K the spin-lattice relaxation is rather fast. At a dopant level of roughly 0.1 mole fraction the spin-lattice relaxation is dominated by spin-orbit coupling. In this range the rate increases strongly with dopant concentration and levels off into saturation above a mole fraction of 0.12. These results are explained in terms of external spin-orbit coupling which induces spin relaxation via a modulation of the overlap integrals between the probe molecule and the heavy atom by lattice motions. Most likely the active lattice motions are due to the disorder modes of the glass. The dependence of the relaxation rate on concentration is well described by complex formation.
Introduction Spin states of impurity molecules are very sensitive in probing the dynamics of the lattice in which they are embedded (for a review see ref 1). Whereas in the crystalline materials the lattice dynamics is determined by phonons, it is the so-called two-level system (TLS) disorder modes which play a dominant role in amorphous materials2” It is assumed that some molecules or atoms or groups of them may occupy either of two sites in the configuration space, which are separated by some barrier. It is further assumed that it is only the two lowest energy states in the corresponding double-well potential that determine the dynamics. The relaxation processes which switch the particles or group of particles between the two sites are either tunneling or thermally activated processes, mediated by the lattice phonons (for a review see ref 7 ) . In case the moving particles in the TLS have some interaction with the probe spin, the TLS motion can relax the probe spin very quickly.* This is exactly the situation in glasses. The spin-lattice relaxation of photoexcited triplet states of aromatic probe molecules, like naphthalene or quinoxaline, is so fast that it is impossible to isolate the triplet sublevels even a t temperatures as low as 1.5 K.6 Instead, the fact that the spin-lattice relaxation rate is fast compared to the electronic decay can be (1) Bowman, M. K.; Kevan, L., In Time Domain in Electronic Spin Resonance; Kevan, L., Ed.; Wiley: New York, 1979. (2) Bowman, M. K.; Kevan, L. J. Phys. Chem. 1977,81, 456. (3) Kurtz, S. R.; Stapelton, H. Phys. Rev. Lett. 1979, 42, 773. (4) Stutzmann, M.; Biegelsen, D. K.; Phys. Rev. B Condens. Matter 1983, 28, 6256. (5) Jackson, W. B.; Stutzmann, M.; Tsai, C. C. Phys. Rev. B: Condens. Matter 1986, 34, 54 1986, 34, 63. (6) Gradl, G.; Friedrich, J. Phys. Reu. E : Condens. Matter 1987, 35,4915. (7) Friedrich, J.; Haarer, D.; Angew. Chem. 1984,96,96; Angew. Chem., Int. Ed. Engl. 1984, 23, 113. Friedrich, J.; Haarel, D. In Optical Spectroscopy of Glasses; Zschokke, I., Ed.; Reidel: Dordrecht, 1986; p 149.
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used to determine the spin-lattice relaxation just by measuring the recovery of the phosphorescence after a fast microwave passage through one of the zero-field resonances.8-10 The most important feature of spin relaxation in glasses is the fact that the parameters (e.g., the energy splitting) of the TLS modes are not well-defined but are, instead, distributed over a fairly large range.ll It is this distribution which determines the characteristic temperature dependence of the relaxation rate which was shown to follow a much lower power law in glasses than in crystals.6 One major question in spin relaxation problems is the question of the nature of spin-lattice coupling. In glassy materials, this question is, of course, related to the nature of the TLS modes involved. In our recent work on spin relaxation of photoexcited triplet states we could definitely show that it is not the hyperfine interaction which governs this coupling.6 Instead we argued that the coupling is most likely due to a discrete, rotational motion of the probe molecule within the amorphous host, which modulates the zero-field splitting. Also, the spin-orbit interaction is a possible mechanism for spin relaxation, though it is usually assumed to be very small for pure aromatic hydrocarbons and, hence, was in this context never considered in detail before. The spin-orbit interaction can, however, be increased by some orders of magnitude by introducing heavy atoms into the solvent, for instance, by doping the glass with haloalkanes. This is exactly what we did in order to find out whether there is a possible influence of spin-orbit coupling on the spin relaxation of photoexcited triplet states of aromatic molecules. (8) Zuclich, J.; von Schutz, J. U.; Maki, A. H. Mol. Phys. 1974, 28, 33. (9) Verbeek, P. J. F.; van’t Hoff, C. A,; Schmidt, J. Chem. Phys. Lett. 1977, 51, 292. (10) Verbeek, P. J. F.; Dicker, A. I. M.; Schmidt, J. Chem. Phys. Lett. 1978, 56, 585. ( 1 1 ) Anderson, P. W.; Halperin, B. I.; Varma, C. M. Philos. Mag. 1972, 25, 1
0 1988 American Chemical Society
