J. Phys. Chem. 1995, 99, 94-101
94
N6'-.
Spectroscopic and Theoretical Studies of an Unusual Pseudohalogen Radical Anion' Mark S. Workentin, Brian D. Wagner, Fabrizia Negri, Marek Z. Zgierski, Janusz Lusztyk, Willem Siebrand,* and Danial D. M. Wayner* Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Canada K I A OR6 Received: June 20, 1994; In Final Form: September 22, 1994@
Azide ( N 3 7 reacts with the triplet excited states of acetophenone (AP) benzophenone (BP) and benzil (Bz) by electron transfer in acetonitrile, generating the corresponding radical anion of the ketone and the azidyl radical (N3.). UV-visible and time-resolved infrared (TRIR) nanosecond laser flash photolysis techniques have shown that the azidyl radicals combine with excess azide at the diffusion controlled limit (k = 1.2 x 1O'O M-' s-l) in an equilibrium reaction forming N6*-. This species is characterized by a broad, featureless visible absorption centered at 700 nm (€700 = 8150 f 920 M-' cm-') and an IR band at 1842 cm-'. The equilibrium constant for formation of N6*- at room temperature was determined to be 200 M-' in acetonitrile, ca. 1000 times greater than in water. The temperature dependence of the equilibrium constant yielded a stabilization energy of 4.4 kcal mol-' for N6*- relative to N3* N3-. Quantum-chemical calculations were carried out to provide some insight into the equilibrium structure of N6'- as well as its associated electronic and vibrational transitions. Excellent agreement between the theoretical and experimental IR frequency was obtained. The theoretical results in conjunction with the experimental observations have allowed for a tentative assignment of the structure of this unusual species to a cyclic, "dimeric" structure of D 2 h symmetry with two long N-N bonds and the azidyl monomeric units essentially intact.
+
Introduction Dihalide and pseudohalide radical anions, X2*- (where X is typically Br, C1, I, or SCN), formed by the interaction of a halogen (or pseudohalogen) atom (Y) with the corresponding anion (eq l), are well-known intermediatesin redox c h e m i ~ t r y . ~ - ~
X'
+ x- x;-
(1)
;=?
Studies on these systems have been facilitated by their stability, since these species typically have equilibrium constants on the order of 104-105 In contrast, considerably less is known about the analogous dimer radical of azide, &'-. Although spectroscopic evidence for the existence of N6*- has been available for some yeaq6-10 previous experiments were carried out in aqueous solutions where the concentrations that could be achieved were too low to study the formation or chemistry of this species in any detail. As a result, characterization of N6'- is essentially limited to a broad absorption band at 645 nm and an equilibrium constant for dissociation into an azide ion and the neutral azidyl radical of 0.33 several orders of magnitude lower than that for dihalide and pseudohalide radical anions. In this paper we present some experimental results relevant to the structure, thermochemistry, kinetics, and spectroscopy (UV-vis and IR) of the N6'- radical ion. Recently we demonstrated that this species can be generated in conveniently high concentrations in aprotic solvents such as acetonitrile. In this study, radical cations of substituted arylanthracenes and @-substituted4-methoxystyrenes reacted with azide at diffusioncontrolled rates in acetonitrile by electron transfer to generate azidyl radicals (eq 2 ) , which subsequently formed N6*- in observable quantities by reaction with excess azide. In fact, our experiments indicated that in aprotic solvents the equilibrium constant for N6*- formation is significantly larger than that in water, allowing for the observation of the N6*- absorption band M-'.235
M-',7910
'
@
Abstract published in Advance ACS Abstracts, December 1, 1994.
(here shifted to ca. 700 nm) even for millimolar concentrations of azide. The enhanced stability of N6'- in acetonitrile has allowed for its chemistry to be investigated in greater detail.'*
Although no information is presently available on the structure of N6'-, the structure of neutral N6 has been the subject of a number of quantum chemical s t u d i e ~ . ' ~ This - ~ ~ work was motivated by the potential for energy storage, since the dissociation of N6 into three NZreleases a considerable quantity of energy. Unfortunately, experimental evidence for the existence of metastable N6 is limited to a transient absorption band at 380 nm observed by Vogler et al. upon laser flash photolysis of a cis-[Fe(N3)2(PPh3)2] complex in ethanol at 77 K.22 Of the many possible structural isomers for N6 (which include the allnitrogen analogues of benzene, Dewar benzene, triprismane, benzvalene, bicyclopropenyl, as well as diazide) the two structures considered the most probable are the cyclic hexaazobenzene (hexazine, 1) and the linear diazide, 2. Initial theoretical investigations concentrated on structure 1 due to its potential for stability through aromaticity 14-17,20 although recent studies have shown that open chain forms, such as 2, are lower in energy than 1 (or any of the other isomer^).^^.'^^.'^ The experimental evidence of Vogler and co-workers does not provide sufficient information to establish the stable form. Similarly, the unresolved absorption band at 645 nm assigned to N6*- does not in itself give insights into its structure. However, the observation that relatively high concentrations of this radical anion can be attained in aprotic solvents opens the possibility of characterizing this species by using UV-vis and infrared laser flash photolysis techniques. Comparison with quantum-chemical calculations may provide assignments that are relevant to the structure and may help to understand why this radical anion is atypical of the halogen and pseudohalogen radical anions in its stability and reactivity.
0022-3654/95/2099-0094$09.00/0 Published 1995 by the American Chemical Society
N6'-
Spectroscopic and Theoretical Studies
J. Phys. Chem., Vol. 99, No. I , 1995 95
N //
1
0.25
I
A.
2
4:1,A 0
B.
Results and Discussion
A. NLFP with UV-vis Detection. Irradiation of nitrogensaturated acetonitrile solutions of acetophenone (AP),benzophenone (BP),or benzil (Bz)yields the corresponding triplet of the ketone^.^^,^^ Addition of azide (as solutions of either its tetrabutylammonium or sodium salt) to these solutions quenches the absorption of the triplet at diffusion-controlled rates (ca. 2 x 1O'O M-' s-' in this solvent)." A transient absorption spectrum (maxima at 335, 480, and 700 nm), recorded on a nitrogen-saturated solution of AP and 10 mM azide (sufficient to quench all ketone triplets), is shown in Figure 1A. The 335and 480-nm bands can be assigned to the radical anion of acetophenone (AP-)by comparison to known spectra25 and by virtue of the fact that the absorptions are quenched by oxygen (Figure 1B). The change in the optical density (AOD)and the kinetics of the broad, featureless absorption at 700 nm are unaffected by oxygen (see Figure 1B). This absorption is assigned to &'-, generated from the reaction of the azidyl radical (N37 with excess azide as illustrated in eqs 3-5. sensitizer sensitizer3*
hvlisc
+ N,-
ET
sensitizer3* sensitizer*-
(3)
+ N,'
(4)
(5) Similarly, when BP or Bz are used as the sensitizer, absorptions due to their respective radical anions (BP-or B z * - ) ~are ~ also observed in the presence of azide. However, both BP- and Bz'- have long-wavelength absorptions which overlap with the absorption of &'-. In these cases the absorption of the ketone radical anions can be removed by addition of air or oxygen to the solution, permitting observation of the absorption of N6'alone. The molar extinction coefficient of at 700 nm (€700) can be determined by making the plausible assumption that the yield of N6*- equals the yield of ketone radical anion. By use of known values for the extinction coefficients of AP-,Bz'-, and BP- 254band the relative initial AOD's of the respective ketone radical anions to N6.- at 700 nm, an average value of 8150 f 920 M-' cm-' has been determined for €700 of N6*-. This agrees with the values of 8000 and 6600 M-I cm-' estimated by pulse radiolysis studies in aqueous solutions by Alfassi et ~ 1 . and '~ Butler et aZ.? respectively. A number of experimental observations have allowed unambiguous assignment of the 700-nm absorption to N6*-. The transient is formed by electron-transfer reaction of azide with the ketone triplet and is not the radical anion of the ketone (whose absorptions can be quenched by oxygen), nor is it N3*, which has a narrow absorption at 274 nm (in water).2d-26In addition, the insensitivity of the absorption to oxygen is expected for inorganic radical anions. An identical spectrum to that observed here was previously reported from azide quenching of the radical cations of 9- and 10-substituted arylanthracenes and /3-substituted 4-methoxystyrenes by electron transfer in acetonitrile, which we also assigned to N6'-.11 Thus, the
d 0.15 0
0.05
0 300
400
500
600
700
800
wavelength/ nm
Figure 1. Transient W-vis absorption spectra of acetonitrile solutions at 23 "C containing acetophenone (1 mM) in the presence of 10 mM tetrabutylammonium azide, measured 1 ps after the laser pulse: (A)
nitrogen saturated solution; (B)oxygen saturated solution. spectrum obtained is independent of the method of formation of azidyl radical, suggesting the transient is indeed formed by a reaction of N3.. Furthermore, the spectrum is similar to the absorption spectrum previously reported for N6*- generated in aqueous solution^,^*^ although in acetonitrile the maximum is red-shifted by ca. 55 nm. Finally, the yield of the 700-nm transient, estimated by the relative AOD at the absorption maximum, increases as the concentration of azide in solution increases. This is expected for a species that is generated from a reaction with azide according to the equilibrium expressed in eq 5. The dependence of the yield of N6'- on the concentration of azide can be described according to eq 6,where AOD is the
optical density at 700 nm at a given azide concentration, a is a constant which includes the AOD at infinite azide concentration, and K5 is the equilibrium constant described by eq 5. A plot of AOD-' vs [N3-]-', illustrated in Figure 2a, gives a straight line whose intercept-to-slope ratio yields an estimate for K5 of 200 M-' at room temperature, Le., approximately 3 orders of magnitude greater than for the same equilibrium in water. Estimates of K5 obtained at several temperatures between -20 and 60 "C indicate that the equilibrium constant for formation of N6'- changes by a factor of 10 over this range, decreasing with increasing temperature. The variation in the equilibrium constant as a function of temperature can be described by the van't Hoff relationship (eq 7).27 Thus a plot of log(&) versus
-LvI" 2.303RT
AS" +2.303R
log(K,) = -
(7)
inverse temperature (shown in Figure 2b) provides estimates of the thermodynamics of the equilibrium in eq 5 (see eq 7). The values determined from this analysis of the data shown in Figure 2b are A W = -4.4 f 0.3 kcal mol-' and AS" = -5.0 f2.5 cal mol-' Thus, N6'- is stabilized by 4.4 kcal mol-' relative to N3' and N3- in acetonitrile. The small negative value for AS" is not consistent with a tightly bound dimer (which should result in a ASo 5 -20 cal mol-' K-') but rather a weakly bound structure that retains a significant degree of rotational freedom.
96 J. Phys. Chem., Vol. 99, No. I, 1995
Workentin et al. 0.10
60
1
0.06
Q
0.02 0.04
0.00 0
4
a
12
16
Time (vs)
Figure 4. Kinetic traces measured by W-vis LFP at 700 nm for an air-saturated solution of acetophenone 10 mM tetrabutylammoinium azide in acetonitrile at 23 "C without added quenchers (e)and in the presence of 10 mM 1,l-diphenylethylene (0).
+
2
2.5
8
2.0 1.5
-
1.0 2.5
2.9
3.3
3.7
4.1
4,s
1OOOwT
Figure 2. (a) Double-reciprocal plot of AOD-' measured at 700 nm versus [N3-]-I at 0 "C. (b) van? Hoff plot (log(K5) versus inverse temperature) for values of K5 determined from the intercept to slope ratio of a series of plots as shown in Figure 2a, which give AHO = -4.4 f 0.3 kcal mol-' and AS" = -5.0 f 2.5 cal mol-' K-l. Error bars represent the estimated error in the determination of 4.
'
0.0
kobs = ko
+ kapp[olefin]
(9)
N6*- in the absence of olefin.29 Values for kappdetermined in this way for 1,l-diphenylethylene and /3-methylstyrene are (4.3 f 0.3) x lo7 and (1.6 f 0.2) x lo7 M-' s-l, respectively. We suggest that the quenching of N6'- by olefins is due to removal of the azidyl radical from the equilibrium in eq 5 by reaction with the alkene, as illustrated in eq 10 for reaction with 1,ldiphenylethylene. This is consistent with the observation of
n
I 0.0
quenches the N6*- absorption, with the resulting decay becoming fiist-order (see Figure 4).32 Plots of the observed rate constant for decay (kobs) of N6*- versus the concentration of added olefin at constant [azide] are linear. The slope of these plots gives an apparent second-order rate constant (kapp)for reaction according to eq 9, where is the sum of rate constants for the decay of
n
I 0.5
1.0
1.5
2.0
2.5
[ N i l / l V 3M
Figure 3. Plot of k g r O d for the formation of Ns*- versus [Ns-] according to eq 8 measured in acetonitrile at -20 "C. Inset shows a representative kinetic trace showing formation of N6*-.
At room temperature the formation of N6*-, under conditions where its absorption could be detected, appears to be instantaneous. However, at lower temperature (-20 "C) the formation of N6'- can be monitored as illustrated in the inset of Figure 3. The growth kinetics of N6*- (kP& varies as a function of azide concentration according to eq 8, where k5 is the rate constant
k g r o=~ ko + ksrN3-1
(8)
for the formation of N6*- (forward reaction, eq 5 ) and ko is related to the intrinsic lifetime of the azidyl radical in the absence of azide.29 A plot of kgrowthvs [N3-] measured at -20 "C is shown in Figure 3; the slope of this plot yields a value of k5 = 6.8 x lo9 M-l s-l. Assuming a low activation energy for N3* N3- combination (2 kcal mol-'), ks at room temperature is estimated to be 1.2 x 1Olo M-l s-l. This value is in good agreement with the value of 1.6 x 1OloM-' s-l reported recently for Br2'- in MeCN30 and those reported for Br2.- (9 x 109 M-' s-1 ), Cl2'- (8 x lo9 M-' s-l), 12'- (1.1 x 1O1O M-l s-l), and (SCN)2'- (9 x lo9 M-l s-l) in water.31 This estimate for k5 at 23 "C leads to a value for k-5 of 5 x lo7 s-l for the unimolecular fragmentation rate constant for N6*-. The absorption of N6'- in this system decays with approximately second-order kinetics with a half-life (t112) of ca. 8 p s at 23 "C (see Figure 4). Addition of an olefin (such as 1,l-diphenylethylene or P-methylstyrene) to the solution
+
the growth of an absorption at 330 nm due to the product radical formed by addition of azide to 1,l-diphenylethylene under nitrogen saturated condition^.^^ As expected for such a transient, addition of oxygen quenches the absorption of the diphenylmethyl type radical through formation of the corresponding peroxyl radical, which does not have an observable absorption in this region. If the quenching of the absorption of N6.proceeds by the loss of N3' by addition to olefins, as illustrated in eq 10, then kapp(eq 9) can be written as expressed in eq 11.
kobs (eq 9) on the concentration of azide which is predicted is, in fact, observed, c o d i n g that the quenching of the observed N6*- absorption occurs through the reactivity of the azidyl radical.34 Therefore at a fixed [N3-] values for klo can be calculated from the experimentally determined kappand K5 values. The rate constant for reaction 10, klo, and that for the analogous reaction with P-methylstyrene are 8.6 x lo7 M-' s-l and 3.2 x lo7 M-' s-l, respectively. These results suggest that the absorption of N6'- can be used to obtain rate constants for the reaction of the azidyl radical and thus serve as a probe for its reactivity.12 B. Time-Resolved Infrared Detection of N6*-(TRIR). The transient IR spectrum obtained for an air-saturated acetonitrile solution of AP and 10 mM azide in the region 1780-1900 cm-' is shown in Figure 5a. A sharp, well-defined peak is observed, with a maximum at 1842 f 4 cm-'. The kinetic trace at 1842
An inverse dependence of
J. Phys. Chem., Vol. 99, No. I, 1995 97
N6*- Spectroscopic and Theoretical Studies a
b
h
6
w/ 1780
1820
a
I 8.
18MI
1900
0
8 4
v/ cm-'
8
1960
2OW
2040
2080
cm"
0
8
12
16
4
8
m 2050
Time/ ps
2090
2130
2170
cm"
12
16 0
Time/ us
1
2
3
4
Time/ ps
Figure 5. TRlR LFP results for an air-saturated solution of acetophenone (ODs08 = 0.3 in I-mm cell) 10 mM tetrabutylammonium azide in acetonitrile at 23 "C: (a) IR spectrum and (b) transient absorption
Figure 6. TRIR LFP results for an air-saturated solution of benzil (OD,, = 0.3 in 1-mm cell) 10 mM tetrabutylammonium azide 80
trace at 1842 cm-' of the formation and decay of (c) IR spectrum and (d) transient absorption trace at 2014 cm-' of the bleaching and partial recovery of Ns-.
mM 1,l-diphenylethylene in acetonitrile at 23 "C: (a) IR spectrum and (b) transient absorption trace at 2108 cm-' of the a-alkyl azide radical product.
cm-' is shown in Figure 5b, and is characterized by an instantaneous growth followed by an approximately secondorder decay. An identical spectrum and kinetic trace were also obtained by using Bz instead of AP as the sensitizer. The kinetic behavior of this absorption (Figure 5b) is identical to the kinetic behavior of the 700-nm absorption (Figure 4, solid circles) measured by UV LFP under the same conditions. Similarly, addition of olefins (1, I-diphenylethylene or j3-methylstyrene) to the solution resulted in quenching of the observed kinetic trace at 1842 cm-', yielding first-order decays. Plots of kobs vs [olefin] were linear and yielded values of the secondorder quenching constants for reaction with the olefin which were in qualitative agreement with those obtained by W LFP quenching studies. However, quantitative results could not be obtained since the IR signals were relatively weak over the limited range of olefin concentrationsthat could be studied. The observed IR band at 1842 cm-' is not due to N3* since its vibration band has been reported at 1658 cm-', with no vibrations in the 1840 cm-' region.35 These results, and their consistency with those from the W LFP studies, strongly support the assignment of the band at 1842 cm-' to &'-. Additional support for the band at 1842 cm-' arising from the reaction of azide can be obtained by monitoring the bleaching of the IR absorption for azide itself, which is known to have an absorption maximum at 2005 cm-' (measured by steady state FTIR of the solution). Figure 5c shows the spectrum obtained for the bleaching of N3- measured under identical conditions to those used above. The very high IR absorption of the solution at the peak of the N3- band resulted in complete attenuation of the IR probe beam in this region and hence the truncation of the peak of this spectrum (thus it appears flat from 1995 to 2014 cm-'). The kinetic trace at 2014 cm-' is shown in Figure 5d and is characterized by an instantaneous bleach followed by a partial recovery of N3-. The rate of recovery of N3- matches the rate of decay of the transient at 1842 cm-', which we have assigned to N6*- (compare parts b and d of Figure 5). The initial bleach of N3- results from the loss of azide in solution due to formation of azidyl radicals in the electron transfer (eq 4) plus an equivalent amount which combines with N3* to form N6*- (eq 5 ) . Although a portion of this initial bleach recovers by regeneration of free azide in the equilibrium,the recovery is not complete, presumably as a result of N3* self-reaction to form neutral N6 (which immediately decays to three N2).
On addition of a number of olefins, the growth of a product having an absorption maximum at ca. 2100 cm-' was observed. Figure 6a shows the IR spectrum and Figure 6b the corresponding kinetic trace of the product observed upon addition of 1,ldiphenylethylene. This peak is characteristic of alkyl azides36 and is assigned to the product of the reaction of azidyl radical to 1,l-diphenylethylene, as illustrated in eq 10. There is no observed effect on the kinetics of this absorption on addition of oxygen, even though the W absorption of the adduct radical was quench.ed with oxygen (vide supra). This is not surprising since the formation of the peroxyl radical should not have a significant effect on the observed IR absorption measured for the alkyl azide stretch. The ability to observe the addition product as well as the effect of added olefin on the recovery of the bleach of azide is consistent with the suggestion (vide supra) that the olefins quench the observed N6*- decay via removal of the low equilibrium concentration of azidyl radicals (eq IO). C. Theoretical Considerations. There are a number of reasons to expect a dimeric structure of the type (N3**-N3).for N6'-. In redox chemistry such dimeric radical anions are well known for halogens (Br, C1, I) and pseudohalogens (SCN).2,3*5-31p37 The rapid establishment of an equilibrium between N3* N3- on the one hand and N6'- on the other, as well as the shift of this equilibrium toward N6*- in less polar solvents, supports this analogy. In all these cases there is a compromise between stabilization by solvation, which favors the monomer in which charge is localized, and stabilization by delocalization, which favors the dimer. Thus, as the solvent polarity increases stabilization by solvation is energetically preferred over that by delocalization. In the gas phase where only delocalization is available the binding energies are expected to be even higher. To test this idea a number of exploratory quantum-chemical calculations were carried out, including ab initio calculations at the Hartree-Fock (HF) level and density-functional (DF) calculations, all based on the 6-3 1+G* basis set and performed with the Gaussian 92 suite of programs.38 Out of a wide variety of structures probed, the four illustrated in Figure 7 were the only structures that gave local minima at the ROHF level. The calculated energies of these structures as well as those of N3' and N3- are listed in Table 1. All other structures tested showed at least one imaginary frequency at the ROHF level. The most stable structure was found to be the planar, symmetric dimer
+
+
+
+
98 J. Phys. Chem., Vol. 99, No. I , I995
Workentin et al.
TABLE 1: Total (Etota,, au (hartrees)) and Relative Energies of Ns*-Structures 7a-d, N3-,and N3' Calculated at the ROHF/ 6-31+G*, UHF/6-31+G*, UMP2/6-31+G*, and the DF-B-LYP/6-31+G* Computational Levels ~~~~
ROHF/6-3 1+G*
UHF/6-3 1+G*
Erelative, kcal mol-'
Etotal,au
UMP2/6-31+G*"
Erelative, kcal mol-'
Etotal, au
DF-B-LYP/6-31+G*
Erelative, kcal mol-'
structure
Etotal, au
7a, D2h
-326.511 775
0.0
-326.530 613
0.0
-327.543 561 (-327.561 699)b
0.0
-326.509 554
+1.39
-326.525 530
+3.20
-327.537 622
+3.74
-326.505 019 -326.445 828
+4.24 +34.78
-327.496 232 -327.464 484
+29.76 +49.84
Etotal, au
Erelative, kcal mol-'
~
7a, GV 7b, C2h 7b, C2 7c, C, 7d, C2v 7 6 Cs
-163.283 295 -163.225 860
N3N3* a
-163.249 912
-163.823 754b -163.697 781b
-328.411 201 -328.411 237 -328.411 020 -328.411 023 -328.360 937 -328.361 318 -328.362 407 - 164.232 015 -164.134 118
0.0
+0.13 +31.56 +30.46
At ROHF/6-31+G* geometry. Fully optimized at the UMP2 level.
F3
2.765 (2,880)
(2.728)
(3.357)
1.136 (1.198)
1.154 (1.202)
7b
7a
H
z
1.174 (1.207)
i
3 (1.397)
~I
1.399 (1.425)
7c
7d
Figure 7. Geometries of the low-energy forms of N6'- calculated at the ROHF and DF (in parentheses) 6-31+G* level. Bond lengths are given in angstroms and angles are shown.where necessary. Angles not shown explicitly are 180" or 90" or equal to given angles by symmetry, as illustrated. IR modes of the most intense vibrations calculated for structures 7a and 7b are illustrated by the arrows.
7a, consisting of two symmetric N3 fragments connected by two long bonds to form a rectangle. An alternative extended planar structure 7b was found to be slightly less stable. Here the two N3 fragments are now asymmetric and connected by a single bond so as to form a Z-shaped structure. Other structures, such as 7c (a cyclic, boat form with dihedral angles of 25.2" and 17.2") and especially the planar s-cis structure 7d, were found to be even less stable. The D6h benzene-like structure (the radical anion analogue of 1) had an energy of +12.12 kcal mol-' relative to 7a (at the HF level) and possessed one imaginary frequency. It thus represents a saddle point on the &*surface which relaxes via a C2,, boat form to structure 7c (C,). The UHF/6-3 1+G* calculations reproduced essentially the same structural parameters and the same ordering as the ROHF calculations; the energy difference between the two structures 7a and 7b was somewhat larger (see Table l), while the degree of spin contamination was comparable for the two forms (9= 0.887 and 0.874 for 7a and 7b, respectively). The DF calculations were performed with the B-LYP f ~ n c t i o n a l . ~Starting ~ from the four ROHF structures, the corresponding DF geometries (data included in Figure 7) were
fully optimized and the spin contaminationwas found to remain within acceptable levels with S2 always being smaller than 0.759. Three of the ROHF structures were found to be saddle points at the DF-B-LYP/6-3 1+G* level, each correlating with two identical minima of lower symmetry: a slightly trapezoidal C2" geometry for 7a, a very slightly nonplanar C2 geometry for 7b, and a planar but asymmetric C, structure for 7d (see Table 1). In the case of 7a and 7b, the calculated barriers of 8 and 0.6 cm-', respectively, are lower than the zero-point energy of the corresponding vibration and hence would have only dynamical significance. For 7d, the barrier is calculated to be 240 cm-'. Since the DF calculationsinclude a considerable amount of correlation, the fact that they predict essentially the same structures as the HF calculations provides significant support for the proposed dimeric geometries. The calculations were extended to include vibrational frequencies and infrared intensities in order to compare the proposed structures with experimental observations. The results are summarized in Table 2. As usual, the HF results require scaling; the scaling factor, determined by comparison of the calculated (2205 cm-') with the observed (2005 cm-') frequency for N3-, was found to be 0.91. The DF-B-LYP frequencies were used without scaling, since this method is known to be reliable for the prediction of vibrational force fields and is found to be accurate for N3* (average error of 21 cm-'). The calculated frequencies for the IR-active vibration are 1743 cm-' (HF) and 1871 cm-' (DF) for 7a and 2021 cm-' (HF) and 1896 cm-' (DF) for 7b,compared to the observed frequency of 1842 cm-'. None of the calculations predict a strong IR band for structures 7c and 7d in this region, so that these structures are unlikely to be responsible for the observed spectrum. Additional calculations of excited state energies and transition moments at the CIS/6-31+G* leve140 (results summarized in Table 3) were only partially successful giving vertical excitation energies of 1.27 (980 nm) and 0.95 eV (1309 nm) for 7a and 7b, respectively. It is difficult to predict the effect of the medium on the electronic absorption and thus compare the experimental and calculated transitions directly since the binding energies are solvent dependent. In this regard we note the 55nm red shift in the maximum of the absorption on changing the solvent from water to acetonitrile (vide supra). Unfortunately our instrumentation does not permit measurement of the electronic spectrum above 800 nm. The lowest absorption band for the nondimeric structure 7c, on the other hand, was predicted o be located at 294 nm, well away from the low-energy transition observed experimentally. To calculate the structure of the absorption band, we would have to evaluate the geometry and force field in the excited
J. Phys. Chem., Vol. 99, No. 1, 1995 99
N6'- Spectroscopic and Theoretical Studies
TABLE 2: Vibrational Frequencies (in cm-l) and Infrared Intensities (in Parentheses) of Structures 7a-d at the ROHF/ 6-31+G* and DF-B-LYP/6-31+G* Levels of Theory 7b 7a 7c 7d mode R O W u mode DF-B-LYP mode ROWu mode DF-B-LYP mode ROWa DF-B-LYP mode ROWu mode DF-B-LYP a, 1576(0) a1 1286(0) a, 2101 (0) a 1874(2) a" 1509(24) 1116(57) a1 2135(645) a' lSSO(471) ~~
670 (0) 188 (0) bzu 1914 (2265) 89 (14) b3* 2026(0) 134 (0) bl. 1585 (455) 665 (716) bze 685(0) a. 66 (0) b3. 691 (21) a
542 (2) 81 (1) 1871 (678) 25 (2) 1885 (8) 100 (0) 1284 (14) 533 (1) 560(0) 15 (0) 562(3)
bz
a2
bl
au
b,
b,
1524 (0) 672 (0) 273 (0) 165 (0) 701 (28) 36 (0) 2221 (3341) 1529 (156) 690 (204) 117 (7) 697 (0)
b
1285 (0) 538 (0) 125 (0) 107 (0) 572 (5) 12 (0) 1896 (807) 1284 (17) 540 (5) 71 (1) 570 (0)
694 (0) 641 (0) 560 (68) 288 (0) 1305 (0) 1034 (0) 892 (0) 875 (38) 637 (8) 528 (1) 363 (0)
bl
82
b2
1309 (13) 1027 (144) 645 (3) 175 (0) 2281 (1029) 1168 (3316) 947 (12) 578 (5) 502(0) 208 (0) 316(20)
a"
1090 (92) 793 (136) 505 (2) 143 (2) 1936 (484) 840 (32) 685 (6) 250 (87) 310(0) 123 (1) 273 (7)
Frequencies unscaled.
TABLE 3: Vertical Excitation Energies (E), Wavelengths (A), and Oscillator Strengths (f, of the N6'- Structures 7a-c Using CIS/6-31+G* Wavefunctions" state structure state symmetry E,eV 1,nm f 7s DO 'B3g o.ooO0 980 0.1812 DI 'B2u 1.2654 816 O.oo00 D2 'A" 1.5194 7b
D3 D4 Do
'B1g *Biu 'Bu
DI
'A,
DZ
'Bg 'A,
D3 D4
7c
Do D1
Dz D3 D4 a
a'
1158 (17) 913 (178) 791 (1) 466 (4) 1746 (82) 1254 (1) 1162 (23) 979 (2) 765 (8) 714 (1) 133 (2)
;2 'A" 'A' 'A' 'A'
2.0021 4.3145
619 281
O.oo00 0.0169
O.oo00 0.9468
1309
0.1577
1.3966 1.8165 4.3851 0.0000 2.8056 2.9154 4.2030
888 683 283
O.oo00 O.oo00 0.0102
442 425 295
O.oo00 0.0010 0.0150
4.2139
294
0.0318
Calculated at optimized ground-stategeometry. Oscillator strengths.
-SOMO
state.41 Since the excitation is found to be basically a HOMO transition which transfers an electron from a bonding to an antibonding orbital, the excited state is expected to be dissociative. In that case we can use the reflection approximation to obtain an estimate of the structure of the absorption band.42 The excited-state surface is approximated by a linear repulsive potential with a slope deduced from the gradients of the excited-state surface at the ground state geometry. In the case of 7a, we find that the repulsive gradient is strongly localized in a single normal mode, namely, the ag mode representing symmetric stretching of the two long N-N bonds. The corresponding gradient is -0.108 hartreemohr. If the squared ground-state vibrational wavefunction corresponding to this mode (calculated to have a frequency of 186 cm-l) is reflected off this linear potential, it generates a Gaussian absorption band with a full-width at half-maximum of about 3500 cm-'. This band width is similar to that observed experimentally if twice the half-width at half-maximum measured on the high energy side of the absorption maximum is used as an estimate to determine the full-width at half maximum value (ca. 4760 cm-'). Hence the structure of the absorption band is not inconsistent with a dimeric structure dissociating into its original two components upon excitation. Further, the red shift in the maximum calculated for the gas phase (980 nm for 7a) relative to solution (700 nm) is consistent with the observed solvent polarity effect on I,, (vide infra). From the analysis of the data in acetonitrile solution in terms of the van't Hoff relation, N6*- was determined to be stabilized by 4.4kcal mol-' relative to N3' N3-. As pointed out above, the stabilization energy is lower in aqueous solution and should
+
be higher in the gas phase. In view of the probable presence of long bonds, it is unlikely that calculations at the HF level are adequate for predicting the binding energy, which indeed comes out negative. The DF calculation yields a stabilization energy of 28.3 kcal mol-' (uncorrected for basis set superposition error), indicating that electron correlation is important for the stability of N6*-. Additional UMP2/6-31+G* level calculations (also summarized in Table 1) confirmed this conclusion, with the stabilization energy for N6*- increasing from 1.64 kcal mol-' at the HF level to 25.2 kcal mol-' at the MP2 level. Full MP2 optimization was carried out only for structure 7a; the corresponding calculation for 7b did not converge to an optimized geometry. The optimized 7a structure differed only slightly from that calculated at the HF level, mainly in a shortening of the long bonding interaction (2.613 A). Since there is little change in the bonding in the two monomeric type units, the effect on the vibrational frequency of the mode strongly active in the IR is expected and indeed found (vide supra) to be minimal. Based on this observation single-point MP2 calculations were performed for the HF equilibrium structures of 7a-d. The resulting total energies (Table 1) were on the same order as in the HF calculations and very close to DF-B-LYP results: structure 7a being the lowest, with 7b close and 7c and especially 7d considerably higher. Attempts to calculate the vibrational force field seminumerically at the MP2 level failed because of an instability of the energies along geometrical displacements. The convergence problems encountered for the two dimeric structures 7a and 7b are probably due to the relatively large spin contamination of the wavefunctions. The charge distribution data obtained suggest that the charge is most delocalized in 7a, Le., approximately equally distributed among the four terminal nitrogen atoms. It is well established that there is a significant solvation energy effect on the localized azide ion on going from acetonitrile to water (ca. 0.5 eV).2,43 The relative solvation energies for the more delocalized species are expected to be smaller,'" thus accounting for the observed solvent effect on K5 in these two solvents. By contrast, the charge in 7b is not significantly delocalized in comparison to N3-. Summary
The radical anion N6*- has been characterized in acetonitrile solution by using a combination of UV-vis and TRIR LFP techniques. The broad long-wavelength absorption is consistent with a dissociative electronic transition in which the dimer fragments into its two monomeric components as has been observed for other halogen and pseudohalogen radical anions.31 There is excellent agreement between the experimental IR band and those calculated for the two lowest energy structures at
100 J. Phys. Chem., Vol. 99, No. 1, I995 various levels of theory (7a and 7b). The observed IR band is indicative of a species in which the bond order is intermediate between that of N3* and N3- (viz., the observed band at 1842 cm-' is within 15 cm-' of the median between the constituting monomers N3- (2005 cm-') and N3' (1652 ~ m - l ) ~ ~While ). no single calculated parameter allows an unequivocal assignment of the preferred structure for N6'- in solution, overall the theoretical calculations favor the cyclic dimer 7a, which possesses long bonding interactions between the four terminal nitrogens of the two monomers. However, it is likely that in solution the molecule will exist in a dynamic equilibrium between 7a and 7b since the calculated energies are close. This is consistent with the small entropy change for its formation (ca. -5 cal mol-' K-l), which indicates the dimer retains significant rotational freedom with respect to the monomers. The higher stability of N6'- in acetonitrile, compared to aqueous solution, suggests that the stability of the dimer may be further increased in even less polar solvents. Furthermore, the ability to quench the absorption of N6*- via removal of N3* from the equilibrium (illustrated by the addition of olefins) provides a useful probe of the kinetics of the azidyl radical, which does not absorb appreciably above 300 nm.I2 Experimental Section General. Ketones utilized as sensitizers were commercially available (Aldrich) and were purified by recrystallization or distillation prior to use. Tetrabutylammonium azide (TCI) was used as the source of azide; however, identical results were obtained with NaN3, Olefins used as quenchers were available from Aldrich ('99%) and were purified by passing them through activated alumina immediately prior to use. Solvents were spectroscopic grade and were used without further purification. Steady-state IR measurements were made on a Nicolet 20BDX Fourier-transform infrared detector interfaced with a NICOS operating system. W-vis spectra were recorded on a Varian Cary 3 spectrophotometer. Nanosecond Laser Flash Photolysis Experiments (NLFP). Nanosecond laser flash photolysis experiments employed the pulses from a Lumonics EX-530 excimer laser (308 nm; 8-ns pulse width; ca. 45 &/pulse) and a computer-controlled detection system previously de~cribed.4~Solutions typically contained an appropriate amount of the ketone sensitizer such that the optical density at the excitation wavelength was between 0.4 and 0.6 (typically 1 mM). To these solutions azide was added directly as the neat solid or as a standard solution in acetonitrile. Samples were contained in rectangular 7 x 7 mm Suprasil quartz cells. Transient absorption spectra were measured employing a flow system which ensured that a fresh volume of sample was irradiated by each laser pulse. Spectra were recorded by using N2, 0 2 , and air-saturated solutions. The absorbance of N6'- was unaffected by oxygen, therefore the quenching studies were performed by using air-saturated solutions with 10 mM azide. Time-Resolved Infrared (TRIR)Experiments. Solutions for the TRIR measurements (prepared to give an optical density of 0.3 at 308 nm) were flowed through a I-mm path length CaF2 cell. The excitation source was a Lumonics Excimer500 laser (XeC1; 308 nm; 10 ns pulse width). The IR probe source was a Mutek Model MPS-1000 diode laser (2000-2280 cm-') or a home-built CO laser (1540-2000 cm-l). Kinetic traces at a particular IR frequency were obtained by measuring the IR intensity through the sample before, during, and after absorption of the UV laser pulse. Response time of the IR detector was ca. 250 ns. Absorption of the excitation pulse caused a shockwave in the kinetic traces, which was dependent
Workentin et al. on the solution and path length. All kinetic traces were therefore corrected by subtracting the shockwave measured at an IR frequency at which no transient was observed. Spectra were obtained by measuring the individual kinetic traces at 4-10cm-' increments throughout the region of interest and plotting AOD vs wavenumber for a fixed time after the 308-nm laser pulse. Details of the TRIR system have been given elsewhere." References and Notes (1) Issued as NRCC No. 37294. (2) (a) de Violet, Ph. Fomier Reviews of Chemical Intermediates 1981, 4, 121. (b) Schuler, R. H.; Patterson, L. K.; Janata, E. J. Phys. Chem. 1980, 84,2088 and references cited therein. (c) Behar, D.; Bevan, P. L. T.; Scholes, G. J. Phys. Chem. 1972, 76, 1537 and references cited therein. (d) Neta, P.; Huie, R. E.; Ross, A. B. J. Chem. Phys. Re$ Data 1988, 17, 1027. (3) (a) Posner, M. L.; Adams, G. E.; Wardman, P. J. Chem. SOC. Faraday Trans. I 1976, 72,2231. (b) Land, E. J.; Priitz,W. A. Int. J. Radiat. Biol. 1977, 32, 203. (c) Bisby, R. H.; Cundall, R. B.; Davies, A. K. Photochem. Phorobiol. 1978, 28, 825. (d) Adams, G. E.; Redpath, J. L.; Bisby, R. H.; Cundall, R. B. J. Chem. SOC.Faraday Trans. I 1973, 69, 1608. (4) For examples, see: (a) Guha, S. N.; Moorthy, P. N.; Mittal, J. P. Radiat. Phys. Chem. 1992, 39, 83. (b) Steenken, S.; Sundquist, A. R.; Jovanovic, S. V.; Crockett, R. Sies, H. Chem. Res. Toxicol. 1992, 5, 355. (c) Heeles, P. F.; Deeble, P. J.; Kim, S. T.; Sara, H. In?. J. Radiat. Biol. 1992,62,137. (d) Abedinzadeh, Z.; Gardes-Albert,M.; Ferradini, L. Radiat. Phys. Chem. 1992,40, 551. (e) Cullis, P. M.; Davis, A. S.; Malone, M. E.; Podmore, I. D.; Symons, M. C. R. J. Chem. SOC.,Perkin Trans. I1 1992, 1409. ( f ) Jovanovic, S. V.; Steenken, S. J. Phys. Chem. 1992, 96, 6674. (5) (a) Mamou, A.; Rabani, J.; Behar, D. J. Phys. Chem. 1977, 81, 1447. (b) Adams, G. E.; Boag, J. W.; Michael, B. D. Proc. Chem. SOC. 1964,411. (c) Baxendale,J. H.; Bevan, P. L. T.; Stott, D. A. Trans. Faraday Soc. 1968, 64, 2389. (d) Kim, K. J.; Hamill, W. H. J. Phys. Chem. 1976, 80, 2320. (e) Kim, K. J.; Hamill, W. H. J. Phys. Chem. 1976, 80, 2325. (6) Singh, A.; Koroll, G. W.; Cundall, R. B. Radiat. Phys. Chem. 1982, 19, 137. (7) Butler, J.; Land, E. J.; Swallow, A. J.; Priitz, W. Radiat. Phys. Chem. 1984, 23, 265. (8) Hurley, J. K.; Linschitz, H.; Treinin, A. J. Phys. Chem. 1988, 92, 5151. (9) Ram, M. S.; Stanbury, D. M. J. Phys. Chem. 1986, 90, 3691. (IO) Alfassi, Z. B.; Prutz, W. A.; Schuler, R. H. J. Phys. Chem. 1986, 90, 1198. (11) Workentin, M. S.; Schepp, N. P.; Johnston, L. J.; Wayner, D. D. M. J. Am. Chem. SOC.1994, 116, 1141. (12) Workentin, M. S . ; Wagner, B. D.; Lusztyk, J.; Wayner, D. D. M. J. Am. Chem. Soc., in press. (13) Dagani, R. Chem. Eng. News 1982, (May) 34 and references cited therein. (14) Saxe, P.; Schaefer III, H. F. J. Am. Chem. Soc. 1983, 105, 1760 and references cited therein. (15) Glukhovtsev, M. N.; von Ragu6 Schleyer,P. Chem. Phys Lea 1992, 198, 547. See addition and correction in Chem. Phys Lett. 1993,204,394. (16) (a) Engelke, R. J. Phys. Chem. 1992,96, 10789. (b) Engelke, R. J. Phys. Chem 1989, 93, 5722. (17) (a) Engelke, R. J. Phys. Chem. 1990,94,6924. (b) Nguyen, M. T. J. Phys. Chem. 1990, 94, 6923. (18) (a) Ramek, M. J. Mol. Strucr. (THEOCHEM) 1984, 109, 391. (b) Huber, H.; Ha, T.-K.; Nguyen, M. T. J. Mol. Srruct. (THEOCHEM) 1983, 105, 351. (19) Ha, T.-K.; Nguyen, M. T. Chem. Phys Lett. 1992, 195, 179. (20) Janoschek, R. Agnew. Chem. Int. Ed. Engl. 1993, 32, 230. (21) Lauderdale, W. J.; Stanton, J. F.; Bartlett, R. J. J. Phys. Chem. 1992, 96, 1173. (22) Vogler, A.; Wright, R. E.; Kunkely, H. Agnew. Chem. Int. Ed. Engl. 1980, 19, 717. (23) Handbook of Organic Photochemistry; Scaiano, J. C., Ed.; CRC Press: Baco Raton, FL, 1989. (24) Murov, S. L. Handbook of Photochemistry; Marcel Dekker Inc.: New York, 1973. (25) (a) Shida, T. Electronic Absorption Spectra of Radical Anions; Elsevier, Amsterdam, 1988. (b) Shida, T; Iwata, S. J. Phys. Chem. 1974, 78, 74 1. (26) (a) Alfassi, Z. B.; Schuler, R. H. J. Phys. Chem. 1985, 89, 3359. (b) Treinin, A.; Hayon, E. J. Chem. Phys. 1969, 50, 538. (27) Benson, S. W. Thermochemical Kinetics; John Wiley and Sons: New York, 1976. (28) Errors quoted are &to. (29) The reciprocal of ko represents an average lifetime of the species in the absence of added quenchers. This may include first order or pseudo first-order (e.g., rearrangements, fragmentations, or radical-molecule
N6*-
J. Phys. Chem., Vol. 99,No. I , 1995 101
Spectroscopic and Theoretical Studies
reactions)and second order (e.& radical-radical reactions)terms. However, only in cases where ko is on the same order as k&s will this term have a significant effect on the kinetic analysis. (30) Scaiano, J. C.; Barra, M; Krzywinski, M.; Sinta, R.; Calabrese, G. J. Am. Chem. SOC. 1993,115, 8340. (31) Nagarajan, V.; Fessenden, R. W. J. Phys. Chem. 1985, 89, 2330. (32) In the cases of AP and BP addition of olefin also quenches the yield of N6.- due to the olefin quenching the triplet precursor. This is not observed when Bz is used as the sensitizer since its triplet energy is lower than the alkenes used (ref 23). Therefore the k,, reported are from airsaturated solutions where Bz was used as the sensitizer. (33) McPhee, D. J.; Campredon, M; Lesage, M.; Griller, D. J. Am. Chem. SOC. 1989, 111, 7563. (34) The alternativereaction would involve addition of N6'- to the alkene followed by dissociation to give the product of a formal azidyl radical addition. This reaction would not show an inverse dependence on [N3-]. (35) Tian, R.; Facelli, J.; Michl, J. J. Phys. Chem. 1988, 97, 4703. (36) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli. J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press, Inc.: San Diego, CA 1991. (37) Wilbrandt, R.; Jensen, N. H.; Pasberg, P.; Sillesen, A. H.; Hansen, K. B.; Hester, R. E. Chem. Phys. k f t . 1979, 60, 315. (38) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.;
Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.;Gomperts,R.; Andres, J. L.; Raghavachari,K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, I. A. Gaussian 92 DFT, Revision G2; Gaussian Inc.: Pittsburgh PA, 1992. (39) Becke, A. D. J. Chem. Phys. 1988, 88, 1053. (40) Foresman, J. B.; Head-Gordon, M.; Pople, J. A,; Frisch, M. J. J. Chem. Phys. 1992, 96, 135. (41) (a) Zgierski, M. 2.;Zerbetto, F. J. Chem. Phys. 1993, 98, 14. (b) Zgierski, M. 2.;Zerbetto, F. J. Chem. Phys. 1993, 99, 3721. 142) Herzbere. G. Soectra of Diatomic Molecules: van Nostraud Co.: Princeton, 1959;'b. p 592. " (43) Marcus. Y.: Kamlet. M. J.: Taft. R. W. J. Phvs. Chem. 1988, 92, 361'3. ' (44) Wayner, D. D. M.; Sim, B. A,; Dannenberg, J. J. J. Org. Chem. 1991, 56, 4853. (45) Kazanis, S.;Azarani, A.; Johnston, L. J. J. Phys. Chem. 1991, 95, 4430. (46) (a) Wagner, B. D.; Zgierski, M.; Lusztyk, J. J. Am. Chem. SOC. 1994,116,6433. (b) Neville, A. G.; Brown, C. E.; Rayner, D. M.; Lusztyk. J. J. Am. Chem. SOC. 1990, 113, 1869. JP941529J
