2340
J. Phys. Chem. 1981, 85, 2340-2344
t r ~ m e t r y .Further, ~~ the extent of energy deposition has been found to correlate with the time scale for unimolecular reactions.22 However, studies such as this and those presented elsewhere provide insight into the fundamentals of polyatomic ion/molecule collisions. A greater under(23)F. W. McLafferty, P. J. Todd, D. C. McGilvery,and M. A. Baldwin, J. Am. Chem. SOC.,102,3360 (1980).
standing of the collision phenomenon is required if applications of collision-induced dissociation mass spectrometry are to be further developed.
Acknowledgment. This work was supported by the National Science Foundation (CHE 77-01295 and 7606142). We thank Professor T. F* Moran for comments.
Low-Energy Electronic Transitions of Alkylamines Phaedon Avouris' and Angelo R. Rossll IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598 (Received: January 12, 1981)
We report ab-initio quantum chemical calculations on the electronic transitions of three model alkylamines: trimethylamine, azabicyclo[2.2.2]octane (Abco),and 1,4-diazabicyclo[2.2.2]octane(Dabco) along with NH3for comparison purposes. Our calculations indicate that the low-lying transitions of these amines are Rydberg excitations with no significant valence state mixing, at least at the ground-state equilibrium geometry. Very good agreement is obtained between calculated excitation energies and observed absorption bands. The changes in the excitation energies, term values, and oscillator strengths of the Rydberg transitions of NH3upon symmetric alkylation are rationalized at the MO level. Special emphasis is placed on elucidating the ordering of the excited states of the symmetric diamine Dabco. The through-bond and through-spaceinteraction of the Rydberg orbitals on the equivalent nitrogens of Dabco result in a -0.5-eV splitting of the resulting states with the symmetric combination states lying at lower energies. The one- and twephoton spectra of Dabco are assigned and, hopefully, some of the existing spectroscopic ambiguities regarding this molecule are resolved.
Introduction The spectroscopy and electronic structure of ammonia and the alkylamines has received considerable attention. Both theory and experiment agree that the lowest excited states of NH, arise from promotions of an electron from the lone pair orbital n, to the 3s(a;), 3p, (e'), and 3pz(a2/1) Rydberg orbitals, respectively.' No vafence states have been identified with certainty.' The picture in the case of the amines is less clearly defined, although, by analogy to NH,, the low-lying states may also be expected to be (3s,n) and (3p,n) Rydberg states.l Alternative interpretations involving valence states, or mixed Rydberg-valence states, have also been considered.2-10 In going from NH3 to the alkylamines the absorption spectra are significantly modified both in terms of the position and of the intensity of the electronic transitions. Simultaneously, the spectra of the (nonrigid) alkylamines broaden to the extent that minimal spectroscopic information can be obtained to aid in the assignment of the states. Among the amines, azabicyclooctane (Abco, 3) and 1,4diazabicyclo[2.2.2]octane (Dabco, 4) have received special spectroscopic a t t e n t i ~ n . ~ ~ * "Their - ' ~ rigid cage structure results in well-structured spectra; thus more detailed spectroscopic information can be obtained. Dabco provides an ideal model system for the study of orbital interactions through chemical bonds and through space.16J7 In its ground state the interacting orbitals are the two lone pair orbitals on the two equivalent nitrogens. The interaction results in molecular orbitals which are the symmetric n(+) and antisymmetric n(-) combinations of the lone pair ~~~
~~~
+Departmentof Chemistry, The University of Connecticut, Storrs, CT 06268.
orbitals with n(+) being at higher energy. Higher energy Rydberg orbitals localized on the nitrogens would also interact and split into symmetric and antisymmetric combinations. The strength of such excited-state interactions involving diffuse Rydberg orbitals, and the resulting state ordering, is of considerable interest. Because of the high symmetry (&) of Dabco both o n e - p h ~ t o n ~ and ~~J~J~ two-photon1'J2 spectroscopies have been utilized to locate and characterize its excited states. However, not all of the predicted low-lying transitions have been observed, and there is no consensus about the assignment of the observed transitions. In this paper, we present the results of ab(1) M. B. Robin, "Higher Excited States of Polyatomic Molecules", Vol. 1, Academic Press, New York, 1974,pp 208-15. (2)D. R.Salahub, Theor. Chim. Acta, 22,325 (1971). (3)A. N. Singh and P. S. Prasad, Chem. Phys., 49, 267 (1980). (4)W. Haque, J. Chem. Phys., 67,3629 (1977). (5)Y. Muto, Y. Nakouto, and H. Tsubomura, Chem. Phys. Lett., 9, 597 (1971). (6)C. G. Freeman, M. J. McEwan, R. F. C. Claridge, and L. F. Phillips, Chem. Phys. Lett., 8,77 (1971). (7)A. M. Halpern, Chem. Phys. Lett., 6,206 (1970). (8) A. M. Halpern, J. L. Roebber, and K. Weiss, J. Chem. Phys., 49, 1348 (1968). (9)E.Tannenbaum,E. M. Coffin,and A. J. Harrison, J.Chem. Phys., 21,311 (1953). (10) R. J. Thompson and A. B. F. Duncan, J. Chem. Phys., 14,573 (1946). (11) D.H. Parker and P. Avouris, J. Chem. Phys., 71, 1241 (1979). (12)D.H. Parker and P. Avouris. Chem. Phvs. Lett.. 53.615 (1978). (13jY. Hamada, A. Y. Kirakawa,'andM. Isiboi, J. Mol: Spectrosc.., 47. .. 440 - - - (1973). (14)T. M. McKinney, Spectrochim. Acta, Part A, 25, 501 (1969). (15)D. H. Parker and M. A. El-Sayed, Chem. Phys., 42,379 (1979). (16)R. Hoffmann, Acc. Chem. Res., 4, 1 (1971). (17)R. Hoffmann, A. Imamura, and W. J. Hehre, J . Am. Chem. SOC., 90,1499 (1968). I
\--
-I-
0 1981 American Chemical Society
Electronic Transitions of Alkyiamines
The Journal of Physical Chemistry, Vol. 85,No. 16, 1981 2341
TABLE I: Vertical Excitation Energies ( e V ) for NH3 excitation
theorya
lA1(3s) +- 'A,(n) 'E(3px,,) +- ' A , ( n ) 'A,(3p,) +- IA,(n) IP
6.37 7.88 8.15
Reference 23. Reference 26.
a
theoryb 6.27 7.84 8.21
' Reference 24.
this work
exptd
6.3 7.8 8.4 10.5
6.38 7.90 8.44 10.W
TABLE 11: Experimental and Calculated Vertical Excitation Energies ( e V ) for N(CH,), and Abco N(CH313 this work
transition 'A,(3s) 'A,(n) ' E ( ~ P , , , ) +- ' A , ( n )
5.7 6.2
'A1(3p,) IP
6.1 8.1
+-
Computational Details All calculations reported here were performed with the GAUSSIAN 70 program.18 The basis set used consisted of the minimal STO-3G basis developed by Pople and cow o r k e r ~ . In ~ ~order to describe the Rydberg states, the minimal basis was augmented by two sets of diffuse Gaussian s and p functions.20 The same exponents were used for the s and p orbitals within each set of diffuse functions. The experimental ground-state geometries were used for all molecules when possible.21 For NH3 (1) an A
expta 5.45
this work 5.1 5.6
6.23
Reference 25.
initio calculations on N(CH3),, Abco, and Dabco. For comparison purposes, calculations of the same type are performed on NH3. The purpose of the calculations is to locate and assign, in terms of orbital promotions, the low-lying states of the above amines. The extent of Rydberg-valence mixing is discussed along with the effects of alkylation on oscillator strengths and term values. Special emphasis is given to understanding the electronic states and spectra of Dabco and resolving the existing uncertainties in the spectroscopic literature.
Abco
a
+-
'A,(n)
Reference 9.
5.1 5.7
5.7 7.4
8.5
exptb
7.7
Reference 8.
conform to those obtained from a SCF calculation on the ground state.
Results and Discussion NH3,N(CH3),,and Abco. The level of calculation employed in the present study of NH, is not as rigorous as other more extensive treatments of the Rydberg states of this m ~ l e c u l e .However, ~ ~ ~ ~ ~the results presented here are in good general agreement both the with experiment and the other more extensive theoretical treatments. This can be seen by inspection of Table I in which results for the excitation energies of NH3 are given. These results allow us with some confidence to apply our simplified treatment to larger, more complex molecules, where the more rigorous treatments cannot be applied easily. The calculated vertical excitation energies for two such molecules N(CH,), and Abco along with experimental values are given in Table 11. In general, calculated vertical excitation energies to Rydberg states are in good agreement with experimentally observed transition energies. The lowest energy band at 5.45 eV (vertical) in N(CH3)3has been identified by Tannenbaum et al.9 as a 'A1(3s) lAl(n) transition and by 'A1(n) transition. The correSalahub2 as a lAl(a*) sponding band (5.1 eV) of Abco has been identified by Halpern et ala8as a lAl(a*) lAl(n) transition and by Parker and Avouris12as a 'A1(3s) lAl(n) transition. Our calculations find no evidence for low-lying valence states in N(CH3)3or Abco and support the 3s n assignment. (see also the discussion concerning Rydberg-valence mixing). The second higher energy band of N(CH3)3at 6.23 eV (vertical) was associated by Tannenbaum et al?, Thomson and Duncan,loand Salahub2with a a* n transition. The analogous vertical transition of Abco at 5.1 eV (0,O at 5.42 eV) was assigned by Halpern et al.8 as a Rydberg transition and, in particular, on the basis of the resulting quantum defect of about 0.5, as a 3p n Rydberg transition. Our calculations support this latter Rydberg assignment. Due to the dissymmetry of the core, the 3p orbital is split to a degenerate 3pF, orbital and a nondegenerate 3p, orbital. The core splitting for the two amines is calculated to be small, about 0.1 eV, compared with a core splitting of 0.5 eV for NH3. Because of the computed small core splitting, in the absence of any other information,the experimentally observed bands cannot be further classified as 'E(3p.J lAl(n), 'A1(3p,) IAl(n), or the superposition of the two transitions. The calculated decrease in core splitting on going from NH3 to the larger molecules of N(CH3)3and +
+
+
R:HI
1 I,R=CH3(21
3
4
N-H bond distance of 1.01 A and an angle of pyramidality, a, of 111,842' were used. For trimethylamine (N(CH3),) (2), azabicyclo[2.2.2]octane(Abco) (3), and l,&diazabicyclo[2.2.2]octane (Dabco) (4), the N-C bond distance was taken as 1.47 A while the C-C and C-H bond distances were 1.54 and 1.09 A, respectively. The value of a was 109.47' for N(CH3)3and Dabco as well as the LCCN and LHCH angles. A slightly different value of a and LCCN angles (107') was used for Abco. The LCCC and LCCH,,,, in Abco assumed a value of 114.10O. All excited-state calculations were performed with a configuration interaction (CI) program22written to interface with GAUSSIAN 70. The CI calculations for each state were carried out by single excitations from the highest occupied lone pair orbital (n) of the SCF ground state into the virtual orbitals yielding the configuration of appropriate symmetry. The technique of single excitation CI used in the present work is equivalent to obtaining excited-state SCF wave functions with the constraint that the core orbitals are frozen and (18) W. J. Hehre, W. A. Lathan, R. Ditchfield, M. D. Newton, and J. A. Pople, QCPE, 10, 236 (1973). (19) W. J. Hehre, R. F. Stwart, and J. A. Pople, J. Chem. Phys., 51, 2657 _ (1969). . ~
_
(20) The exponents for the diffuse s and p functions had a value of 0.0257 for the inner set and a value of 0.0111 for the outer set. (21) For NHs: W. S. Benedict and E. K. Plyler, Can.J.Phys., 35,1235 (1957). For N(CH3)3: D. R. Lide and D. E. Mann, J. Chem. Phys., 27, 914 (1958). For Dabco: T.Wada, E. Kichida, Y. Tomhe, H. Suga, S. Seki, and I. Nitta, Bull. Chem. SOC.Jpn., 33, 1317 (1960). (22) M. H.Whangbo, private communication.
-
+
-
+
-
-
(23) R. Rianda, R. Frueholz, and W. A. Goddard, 111, Chem. Phys., 19, 131 (1977). (24) R. Runau, S.D. Peverimhoff, and R. J. Buenker, J. Mol. Spectrosc., 68, 253 (1977). (25) D. W. Turner, C. Baker, A. D. Baker, and C. R. Brundle, "Molecular Photoelectron Suectroscouv". New York. _ - . Wilev-Interscience. 1970, p 357. (26) W.R. Harshbarger, J. Chem. Phys., 54, 2504 (1971).
2342
The Journal of Physical Chemistry, Vol. 85, No. 16, 1981
TABLE 111: Term Values ( e V ) for NH,, N(CH,),, and Abco
TABLE IV: Oscillator Strengths for NH,, N(CH, 1., and Abco
calcd molecule
(3w)
(3Pw.v,n)
4.2 2.4 2.3
2.7 1.9 1.8
Avouris and Rossi
transition
expt (3PZP) (3s,n)
3s f n
(3p,n)
3~,,, n, 3pZ n, calcd calcd f
"3
N(CH,), Abco
2.1 2.0 1.7
4.4 3.0 2.6
2.9(pX,,) 2.3 2.0
Abco correlates with the fact that core splitting is rarely resolved experimentally for first-row molecules with more than 10 atoms.27 The Rydberg states in question are, of course, members of a Rydberg series and their term values (T,)should follow the equation
T, = IP - E,,,
D 1L
=-
( n - 6)2
In eq 1,IP is the ionization potential to which the series converges, E,,, is the excitation energy, n is a positive integer (principal quantum number), R is the Rydberg constant, and 6 is the quantum defect. Table I11 shows the correlation of term values computed by using our theoretical excitation energies and ionization potentials with experimental term values. It can be seen that there is good qualitative agreement between theoretical and experimental values. There is a general decrease in the experimental (35,n) and (3p,n) term values in going from NH3 to the alkylated amines N(CH3)3and Abco which is also mirrored in the calculated term values. By considering Tables 1-111, the decreasing trend in term values upon alkylation appears to be influenced mainly by the corresponding decrease in the ionization potential. For example, there is a decrease in the IP of about 2.3 eV in going from NH3 to N(CH3)3 but the 3s n transition energy decreases by only 0.9 eV. That is, the effect of alkylation is stronger on the lone pair orbital than on the Rydberg orbital. This destabilization of the lone pair orbital by the methyl groups can be understood by considering the methyl groups as pseudo p orbitals interacting with the p component of the nitrogen lone pair (hyperconjugation). This interaction results in a lower energy (bonding interaction) orbital consisting mainly of the alkyl groups and a higher energy (antibonding interaction) orbital consisting mainly of the lone pair. This latter orbital lies at an energy higher than the noninteracted lone pair orbital. In the case of Abco there is a further destabilizing effect of the through-bond interactions similar to those which occur in Dabco (subsequent discussion),raising its lone pair orbital still higher in energy than for N(CH3)3. A comparison between the calculated and experimental oscillator strengths of the transitions discussed above is given in Table IV. The symmetric alkylation of NH3 significantly affects the relative strengths of the 3s n and 3p n transitions. The main trend revealed by the experiment (Table IV) is that the oscillator strength for the 3s n transition is larger than the 3p n transition in NH3, but this trend reverses for N(CH3)3and Abco. The agreement of the calculated oscillator strengths with the experimental results is marginal (only single excitations from the lone pair orbital are considered in this calculation), but the trend in the reversal of the values of the oscillator strengths in going from NH3 to the substituted amines does come through.
-
-
-
-
-
(27) Reference 1, p 12. (28) W. R. Harshbarger, A. Skerbele, and E. N. Lassettre, J. Chem. Phys., 54, 3784 (1971).
molecule "3
N(CH,), Abco
calcd
expt
3~ n, expt +-
0 . 1 1 0 0.07ga
0.000
0.000
0.002
0.005 0.016a
0.015
0.011 0.003b
0.076
0.031 0.029
0.13a 0.06b
(PX,Y)"
Reference 9. Reference 8. Reference 28. Regarding the 3p, +- n transition, see ref 31. a
The oscillator strength trends in NH3 itself can be rationalized in terms of the united atom, in this case Ne, description of the NH3 molecule. As has been pointed out by JungenB and Glownia et the AI = fl selection rule for atoms can carry over quite well to corresponding small molecular systems. Accordingly, the 3s 2p of Ne is allowed (Al = -l),and so is the corresponding 3s n(2p,) transition of NH3. The 3p 2p transition of the united atom is forbidden (A1 = 0), and so are the 3p,, n(2p,) and 3p, n(2p,) transitions in NH3. However, the 3p, n(2p2)transition can acquire intensity by mixing with the allowed (AZ = +1) 3d n(2pJ transition; no such mixing is possible for the 3p, n(2p,) transition. To the extent that the united atom description is applicable to NH,, and since d functions are not included in our basis set, the oscillator strength of the 3 ~ , , ~n transition is expected to be underestimated. We find that in the case of NH3 the (core)(n)'(3s)' and (core)(n)'(3p2)' configurations are relatively pure with small admixture; however, significant mixing takes place between these two configurations upon symmetric alkylation (Table V). As a result of this mixing, the "3s" n oscillator strength will be reduced in N(CH3)3 and transferred to the "3p," n transition (as observed experimentally). This trend is further enhanced by changes in the core orbitals; e.g., the contribution of the 2p, A 0 to the lone pair orbital is reduced by alkylation. Mixing analogous to the 3s 3pz mixing shown in Table V should also exist among the (~ore)(n)~(3p,,,)'and (core)(n)'(3dXz,?) configurations. Of particular importance to the issue of the identity of the excited states is the extent of Rydberg-valence mixing. From Table V it can be seen that there is virtually no mixing of the valence virtual configurations with the lowlying Rydberg configurations for the ground-state equilibrium geometry. The same conclusions hold for the larger molecules Abco and Dabco. The mixing of core orbitals to the Rybderg MO is also found to be negligible (
