4321
J. Phys. Chem. 1992, 96,4321-4324 protonated form of 2,2’-DNA has a similar conformation (and hence is stabilized by similar forces) as found in the singlet excimers of 2,2’-DNE and 2,2’-DNM, formed by the bimolecular annihilation of the corresponding triplet excimers.
Conclusions We have demonstrated that an efficient charge-separation process takes place in symmetric 1,l’- and 2,2’-dinaphthylamines leading to the formation of a highly polar excited singlet state. The effects of the bridge position (1 vs 2) on the degree of charge separation and the rate of dielectric relaxation were also shown. Qualitative arguments were provided to link the observed differences of 1,l’- and 2,2’-DNA to structural factors governing the internal rotation, and symmetry breaking, in these molecules.
The comparison of 2,2’-DNA with its protonated form and with the corresponding methane and ether homologues demonstrates the importance of the amine bridge in the CT process. We plan to complete this study by quantitative determination of the dielectric relaxation rates, measured in a broader range of solvents, for the purpose of separating the viscosity and temperature effects on the observed reaction rate constants. The final objective of such a study is to provide an appropriate physical model for excitation delocalization and charge redistribution in the molecular systems discussed.
Acknowledgment. This work was supported by a grant (DEFG02-89ER14024) from the Office of Basic Energy Sciences of the Department of Energy.
UV Absorption Spectrum and Photolysis Studies of NCI, Radicals in Low-Temperature Argon Matrices Jdanna V. Gilbert* and Brendon D. Hofsetz Chemistry Department, University of Denver, Denver, Colorado 80208 (Received: January 2, 1992; In Final Form: February 10, 1992)
NCI2 radicals were produced and deposited in a low-temperatureargon matrix by passing NC1, in argon through a heated stainless steel needle valve and depositing the effluent onto a cold KCI (10 K) window. By varying the deposition conditions, low-temperatureargon matrices consisting of NC12,of NCI2and NC12 dimers, and of NCI2, NC12dimers, and NCI, were prepared. All species were identified via the FTIR spectra of the matrices. The UV absorption spectrum of NC12 in an argon matrix was measured and consists of diffuse bands with a band separation of 560 cm-’ overlying a continuum. The maximum of the transition is at 298 nm. The matrix spectrum is identical to a transient absorption spectrum reported following flash photolysis of CI2-NCl3gas mixtures and establishes that the lower state of the transition is the ground electronic state of NCI2. Upon broad-band photolysis, the NC12dimers were destroyed, producing NC4 and presumably NCI, but no changes were observed for the NCI2 radicals, suggesting that the C1 NC1 photofragments recombined in the matrix cage.
+
Introduction The halogen aminyls (NX2and NXY; X, Y = halogen) comprise a class of radicals about which almost no information is available. These radicals are certainly involved in the sometimes explosive decomposition of the halogen amines (NX3, NX2Y), but detailed studies of the decomposition mechanisms have not been camed out. Interest in these radicals has been rekindled because of their ability to generate halogen nitrenes (NX) in excited singlet states via chemical reactions.l-1° The energy and long lifetimes of these excited states make the nitrenes useful energy carriers in a variety of chemical laser systems. For example, the possibility of substituting the 02(a1A)in the 02-I laser with a halogen nitrene (which are isovalent with 0,) has been investigated. In the 02-I laser, chemically generated 02(a1A)pumps I* via the reaction 02(a1A)+ I(2P3/2) 02(X3Z) + I*(2Pl/2).11 The analogous
-
Clyne, M. A. A.; White, I. F. Chem. Phys. Lett. 1970, 6, 465. Malins, R. J.; Setser, D. W . J . Phys. Chem. 1981, 85, 1342. Herbelin, J. M.Chem. Phys. Len. 1976, 42, 367. Herbelin, J. M.; Cohen, N. Chem. Phys. Left. 1973, 20, 605. Cheah, C. T.; Clyne, M. A. A,; Whitefield, P. D. J . Chem. Soc., Faraday Trans. 2 1980, 76, 711. Cheah, C. T.; Clyne, M.A. A. J . Chem. SOC.,Faraday Trans. 2 1980, 76, 1543. (6) Koffend, J. B.; Gardner, C. E.; Heidner, R. F . J . Chem. Phys. 1985, (1) (2) (3) (4) (5)
83, 2904. (7) Herbelin, J. M.; Spencer, D. J.; Kwok, M. A. J. Appl. Phys. 1977.48, 3050. .... (8) Heidner, R.F.;Helvajian, J.; Holloway, J. S.;Koffend, J. B. J . Phys. Chem. 1989, 93, 7813. (9) Exton, D. B.; Gilbert, J. V.; Coombe, R. D. J . Phys. Chem. 1991.95, 2692. (10) Exton, D. B.; Gilbert, J. V.; Coombe, R. D. J . Phys. Chem. 1991,95, 7758.
+
-
+
reaction NCl(alA) I(2P3/2) NCl(X3Z+) I*(2Pl/2)has been demonstrated.I2 Until recently, the only known precursors of metastable halogen nitrenes had been the corresponding halogen azidesl3-I5and, in the case of NF, N2F4 as In the N2F4process, N2F4is thermally dissociated to NF2, which is then reacted with H atoms to produce very high yields of NF(alA). Since other halogen analogues of N2F4are not known, the production of NCl(alA) (or other NX*) by this route is not possible. Another source of metastable nitrenes has been demonstrated by the recent work performed in our laboratory, in which significant densities were generated via hydrogen atom reactions with halogen amines.9J0 Specifically, the generation of NCl(alA and blZ+) from H NCl, and of NF(alA and b’Z+) and NCl(alA and blZ+) from H NFC12 has been observed. Kinetic data obtained in these studies were consistent with a two-step mechanism which involved the formation of the aminyl radical in the fmt step and the subsequent reaction of the aminyl radical with a H atom to form the excited-state nitrene. The presence of the aminyl radicals as intermediates in these reactions could not be verified because of the lack of the necessary spectroscopic information for these radicals. In order to study the spectroscopy of the aminyl radicals, methods
+
(11) McDermott, W. E.; Pchelkin, N. R.; Benard, D. J.; Bousek,
+
R. R.
Appl. Phys. Left.1918, 32,469. (12) Bower, R.D.; Yang, T. T. J. Opt. Soc. Am. B 1991,8, 1583. (13) Benard, D. J.; Winker. B. K.: Seder. T. A.: Cohn, R. H. J . Phvs. Chem. 1989, 93, 4790. (14) Coombe, R. D.; Patel, D.; Pritt, A. T., Jr.; Wodarczyk, F . J. J . Chem. Phys. 1981, 75, 2177. (15) Coombe, R. D. J . Chem. Phys. 1983, 79, 254.
0022-3654/92/2096-432 1$03.OO/O 0 1992 American Chemical Society
4322 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
Gilbert and Hofsetz
for generating and then isolating them without other interfering species in low-temperature argon matrices are being developed in our laboratory. This paper reports the production of NCI2from NC13 and the IR and UV absorption spectra of this radical in low-temperature argon matrices. Experimental Section The synthesis of NCl, has been reported previously.16 Briefly, Clz in argon is bubbled through a saturated solution of (NH4),S04 in 1 M H2S04. NC13 is synthesized in the solution and because of its high vapor pressure and low solubility in water, is swept out of the solution with the argon and the excess C1,. NCI3 and ClZ are trapped in a methanol/dry ice bath. After a small amount of NC13 is synthesized, the condensed NC13and C12are warmed to room temperature to allow the Clz to evaporate. The remaining NC13 is recooled with the dry ice bath. Argon is passed over the cold NC13, and NCl, vapor is entrained in the argon flow, with an estimated NC13/Ar ratio of 1:lOOO. This gas is carried through Teflon lines to the cold head where a small amount is admitted to the cold head and the remainder is returned to the hood. The NCl, generator is housed in a fume hood and is well-shielded since explosions can occur. NC12 was generated by passing the NC13/Ar gas through a stainless steel metering valve (Whitey, SS-21RS4) which had been heated by wrapping the valve with heating tape. The temperature was monitored by attaching a J-type thermocouple to the outside of the valve. The effluent from the heated valve was deposited on the cold KCl(l0 K) window mounted in an RMC Cryosystem Model LTS-22 cold head system. FTIR spectra of the matrices produced were collected with a Nicolet Model 5DX-PC FTIR (resolution f2 cm-I). The deposition rates were not measured directly in these experiments; however, clear NC12matrices were obtained when the NC12 IR absorbance at 679 cm-I grew in at 0.015-0.02 absorbance units per 15 min, and the deposition was carried out for about an hour. (Clear matrices gave the best UV absorption spectra.) A scanning monochromator (PTI, 0.25 m), a Xe lamp (PTI, 150 W), and a cooled GaAs photomultiplier tube were used to obtain the UV absorption spectrum in a single-beam experiment. A clean KCl window similar to the window on which the NClz matrix was deposited served as the background "blank", and the background was ratioed with the signal collected from the NC12/Ar matrix to calculate the absorption spectrum. The spectrum was measured from 250 to 380 nm, the limits set by the transmission of the KCI window at the low end and the characteristics of the Xe lamp at the high end. For the matrix photolysis experiments, the grating in the monochromator was set to zero order, and the output of the Xe lamp was filtered with a cylindrical water-filled cell (10 cm long) equipped with quartz windows. The UV photolysis beam overlapped the region of the matrix sampled by the FTIR beam.
Results and Discussion characterization of NC12 in Matrices. In 1981, Kohlmiller and Andrews isolated NC12 in low-temperature argon matrices, prepared by passing N2, C12,and Ar through a microwave discharge and depositing the effluent on a cold ~ i n d 0 w . l Although ~ a variety of species were produced in the discharge and subsequently deposited in the matrix, on the basis of the observed chlorine isotopic triplet and the proper nitrogen-15 isotopic shifts, the NCll asymmetric stretch was assigned at 679 cm-I and the approximate Cl-N-Cl bond angle was calculated to be 111'. To study the spectroscopy of NC12without interference from other species, it was necessary to develop a method of generating NC12 in which the presence of other species was minimized. It had been suggested that metal surfaces can cause NC13 to decompose,l*and indeed, during our matrix isolation studies of NCl3I9we observed that (16) Gilbert, J. V.;Wu, X.L.; Stedman, D. H.; Coombe, R. D. J . Phys. Chem. 1987, 91, 4265. (17) Kohlmiller, C. K.:Andrews, L. Inorg. Chem. 1982, 21, 1519. (18) Wei, M. S.; Current, J. H.; Gendell, J. J. Chem. Phys. 1972, 57, 2431. (19) Gilbert, J. V.;Smith, L. J. J . Phys. Chem. 1991, 95, 7278.
04 m
02
NClp
WAVENUMBERS
Figure 1. FTIR absorption spectrum of low-temperature matrices prepared by passing NC13 in argon through a heated metal metering valve: (A) matrix with NC12 dimers (689 cm-I), NC12 (679 cm-I), and NCI, (644 cm-I); (B) matrix with NC12 dimers and NCI2; (C) matrix with NCI, only.
using a stainless steel metering valve to admit the NCl, to the cold head vacuum region resulted in the appearance of NC12 in the matrix. However, the NC12 IR absorbance stopped increasing after a few minutes, and thereafter, only NCl, was deposited. The assumption was made from these observations that a surface interaction was responsible for the decomposition of NC13 to NC12 and that over time the metal surface was passivated. This observation was exploited for the current study, and to prevent the apparent passivation of the valve, the valve was heated. It was found that when the temperature of the valve was maintained at 75 OC, NC12with no NC13 was deposited in the matrix. At higher temperatures, little or no NC12was deposited, and at lower temperatures both NClZand NC13 were deposited. The rate of deposition was also an important parameter in these experiments, and incomplete conversion of NCI, to NC1, was observed when the deposition rate was too high. No evidence for the production of NC1 was obtained from the IR spectra of the mat rice^,^^*'^*^^ suggesting that it is not formed to any great extent (if at all) via the process at the heated valve. It is, however, probable that atomic and/or molecular chlorine were deposited, neither of which would be observed via the IR spectra. FTIR spectra of three representative matrices are shown in Figure 1. In Figure 1A, features appear at 644, 679, and 689 cm-I, in Figure lB, features appear at 679 and 689 cm-I, and in Figure lC, a single feature appears at 679 cm-'. The peaks at 644 and 679 cm-I are assigned to NC13 and NCl,, respecti~ely.'~.~~ ~
~~
(20) Milligan, D. E.; Jacox, M. E. J . Chem. Phys. 1964, 40, 2461.
The Journal of Physical Chemistry, Vol, 96, No. 11, 1992 4323
NC12 Radicals in Argon Matrices
I
I
270
I
280
I
I
I
I
1
290 300 NANOMETERS
1
I
310
320
Figure 2. UV absorption spectrum of NCI, in a low-temperature argon matrix. TABLE
I Band Positions (nm) in the NC12 UV Absorption Spectrum Briggs Briggs
(h0.3nm)
and NorrishZ9
283.5 290.0 293.5
215.5 219.4 283.5 289.0 293.5
this work
this work
(h0.3nm) 298.0 303.0 308.0 314.0
and Norrishzg
291.9 303.8 308.5 314.1
A peak at 689 cm-' was also observed in the Kohlmiller and Andrews study and had been tentatively assigned to NClz dimers." Our experiments verify this assignment since the 689-cm-I peak was only observed when the NCl, peak was observed and in matrices prepared with somewhat high deposition rates. The alternative suggestion assigning this feature to NCl dimers is not consistent with our experiments since no NCl is observed in any of the matrices produced with the heated needle valve. W Absorption Spectrum of NCI, Radicals. The UV absorption spectrum of NC12 in a low-temperature argon matrix is shown in Figure 2. Because an IR spectrum of the matrix was also measured, the assignment of the UV spectrum to NClz is unequivocal. The spectrum consists of diffuse bands, possibly overlying a continuum, and extends from about 270 to 320 nm. The energies of the bands (calculated from the wavelength of the center of each band) are listed in Table I. NFz is the only other dihalogen aminyl radical for which an absorption spectrum has been r e p ~ r t e d . ~ lThe - ~ ~NF2 spectrum is shifted to shorter wavelengths (maximum at 260 nm) than the NC12 spectrum, but is similar in general appearance to the NClz spectrum, consisting of diffuse bands overlying a continuum. The band separation in the NF2 spectrum is 380 cm-I and has been assigned to the u2 bending mode of the NFz excited state. The band separation of the NClZUV absorption spectrum is 560 cm-I. This value, however, seems too large to be assigned to the bending mode of the excited state of NClz and is instead assigned to a stretching mode of the NClz excited state. The assignment of the 560-cm-I frequency to a stretching rather than the bending mode is reasonable when compared to the uz value (which has not been measured) estimated here for ground-state NC12. Under the assumption that the ratio of the ground-statevalues for u2(NCl2) and u2(NC1,) is similar to the ratio of the corresponding modes in NFz and NF3 (573 and 649 cm-l, r e s p e c t i ~ e l y ~ and ~ * ~using ~), 365.2 cm-I for the NC13 u2 frequency (from the IR spectrum of NC1, in a low-temperature nitrogen matrixz6),a value of 322 cm-I for the NClz bending mode is obtained. The involvement of a stretching mode implies that the excited state of this UV transition (21) Goodfriend, P. L.; Woods, H. P. J . Mol. Spectrosc. 1964, 13, 63. (22) Jacox, M. E.; Milligan, D. E. J . Mol. Spectrosoc. 1974, 52, 322. (23) Heidner, R. F., 111; Helvajian, H.; Koffend, J. 9. J . Chem. Phys. 1987,87, 1520. (24) Harmony, M. D.; Myers, R. J. J . Chem. Phys. 1962, 37, 636. (25) Shamir, J.; Hyman, H. H. Spectrochim. Acta 1967, 2 3 4 1899. (26) Sawodny, W.; Hartner, H. J . Mol. Srrucr. 1989, 213, 145.
is Franck-Condon shifted with respect to the ground state. In pulsed laser photolysis experiments of gas-phase NCl3,I6 banded visible emission was observed with a band separation of 320 cm-I. This emission was tentatively assigned to NCl,, based on the information available at the time (Le., the uz frequency for NC13 in CC14solution27). However, the value estimated above for uz of NClz (322 cm-l) suggeststhat, in fact, the visible emission was from NC12. If this is so, then the UV absorption spectrum of Figure 2 is not a transition to the lowest lying excited electronic state of NC12. This conclusion is consistent with predictions made via the Walsh diagram for AB2 species with 19 valence electrons2* (such as these NX2 radicals). The diagram predicts that the ground state is bent and that the first excited state is linear. A transition between the ground and first excited state is, therefore, likely to involve a bending mode, consistent with the analysis of the NFz UV transition at 260 nm and with the visible emission observed in the gas-phase NCl, studies. The NCl, UV transition shown here does not appear to be dominated by a bending mode. Absorption spectra from reactive NC13 systems have been reported and now can be unambiguously assigned in light of the UV absorption spectrum of NC12 shown here. In 1963 Briggs and Norrishz9observed a transient absorption spectrum following the flash photolysis of gas-phase mixtures of C12and NC1,. The spectrum consisted of diffuse bands overlying a continuum and was assigned to NC12 since there were no other likely sources. Later, in studies of the C1 NC13 reaction, Clark and Clyne30 assumed that an absorbance observed between 280 and 3 10 nm was due to NC12 (based on the Briggs and Norrish assignment). In both of these cases, however, no additional evidence was obtained that could verify the assignment of the spectrum. Table I lists the band centers reported by Briggs and N o r r i ~ hand , ~ ~as can be seen by comparison with our data, their spectrum is virtually identical to ours. In addition, the lower state of the transition can be definitely assigned to the ground electronic state of the radical, since our spectrum was obtained for NClz in a low-temperature environment. Photolysis of NClz Matrices. The diffuse band structure observed in the NClz UV absorption spectrum implies a predissociated state, while the underlying continuum either may be a separate transition to a dissociative state or simply reflect the inability to resolve overlapping bands. Photolysis in this UV absorption band should, in either case, cause the NC12radical to fragment. Broad-band photolysis of argon matrices containing NClZ,however, caused no change in the NC12 IR peak at 679 cm-l. This apparent lack of change in the NC12 peak implies that NC1 and C1 photofragments formed recombine in the matrix cage since it is extremely unlikely that either fragment can escape the argon matrix cage.I9 Photolysis of matrices containing NClz dimers caused the dimer peak at 686 cm-' to decrease with a corresponding increase in the NCl, peak at 644 cm-I, as is shown clearly in Figure 3. The decrease in the NC12 dimer peak at 689 cm-I and the corresponding increase in the NC1, peak at 644 cm-' are consistent with the process [NClz]2 hv NC13 NCl (1)
+
+
+
+
We assume that no NCl was observed during the NC12 dimer photolysis because NCl has a very small dipole moment and hence is a very weak IR absorber. From the IR absorption intensities in our previous NC1, matrix experiments,19the NCl IR absorption cross section at 824 cm-I is estimated to be less than 20%of the NCl, IR absorption cross section at 644 cm-I. In Figure 3A,B, an increase of 0.04 absorbance units was observed in the NCl, peak as a result of the NC1, dimer photolysis. Under the assumption that the product ratio of NCl to NC13 is 1 (as predicted in reaction l), then the corresponding NCI absorbance would be less than 20%of 0.04, that is, less than 0.008absorbance units. (27) Hendra, P. J.; MacKenzie, J. R. Chem. Commun. 1968, 760. (28) Walsh, A. D. J . Chem. SOC.1953, 2266. (29) Briggs, A. G.; Norrish, R. G. W. Proc. R. SOC.London 1964, A278, 27. (30) Clark, T. C.; Clyne, M. A. A. Trans. Faraday SOC.1969,65,2994.
4324 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
Gilbert and Hofsetz when the photolysis was continued until all of the dimers were completely photolyzed, the NC13 peak at 644 cm-l was finally observed to decrease, and in the matrices in which enough NC13 was photolyzed, a weak NCl peak at 824 cm-’ appeared.17J9q20
WAVE NUYBE RS
Figure 3. FTIR absorption spectra of a low-temperature argon matrix containing NCll dimers (689 cm-I), NC12 (679 cm-I), and NCI, (644 an-’):(A) before and (B) after 30 min of broad-band UV/vis photolysis.
Our previous photolysis studies of NC13/Ar matrices demonstrated clearly the production of NCI from the photolysis of NCl3.I9 The results shown here show, however, that when both NCI2 dimers and NC13 are present in the matrix, the production of NC13 from NCl2 dimer photolysis is more efficient than the destruction of NC13from NC13photolysis. In several experiments,
Conclusions The successful isolation of NC12 in a low-temperature argon matrix without other UV or IR absorbing species has made it possible for us to obtain the UV absorption spectrum of NC12. The UV absorption spectrum consists of diffuse vibrational structure (band separation 560 cm-I) possibly overlying a continuum. The diffuse band structure of the spectrum is indicative of a predissociated state, and so, NCl, is expected to fragment under UV irradiation. UV photolysis of these matrices, however, reveals that NC12does not undergo permanent fragmentation in the matrix, whereas NCI2dimers photolyze to produce NCI, and presumably NCl. The photolysis behavior of NC12 in a matrix suggests that the excited states of this radical can be probed by irradiating the matrix with a pulsed UV laser and analyzing the emission that may result from the C1 + NCI recombination in the matrix cage, a method that has been applied successfully to study excited states of other matrix-isolated species.” Finally, the ability to spectroscopically characterize the halogen aminyl radicals promises to yield a wealth of information about the radicals themselves as well as about the energetic amines and their reaction mechanisms. Acknowledgment. B.D.H.was supported by the National Science Foundation, Research Experience for Undergraduates Grant CHE-900078 1. This work was supported by the donors of the Petroleum Research Fund, administered by the American Chemical Society, and by the National Science Foundation under Grant CHE-89 10143. (31) For example: Bondybcy, V. E.; Fletcher, C. J . Chem. Phys. 1976, 64, 3615. Mandich, M.; Beeken, P.; Flynn, G. W. J . Chem. Phys. 1982, 77, 702.
