J. Phys. Chem. 1984,88, 1601-1605
1601
Electron Spin Resonance Investigation of the 5-Halouracil and 5-Halocytosine .;rr-Cations Produced by Attack of the CI,- Radical Michael D. Sevilla,* Steven Swarts, Department of Chemistry, Oakland University, Rochester, Michigan 48063
Heinz Riederer, and Jiirgen Hiittermann Znstitut fur Biophysik und Physikalische Biochemie, Universitat Regensburg, Regensburg, 0 - 8 4 0 0 , West Germany (Received: August 8, 1983)
The a-cation radicals of a number of 5-halopyrimidines have been produced by attack of the one-electron oxidizing agent, Cl,, in y-irradiated basic 12 M LiCl glasses at low temperatures. Analysis of the ESR spectra for the a-cations was completed with the inclusion of nuclear quadrupole terms for C1, Br, and I, in addition to the hyperfine and g tensors. The halopyrimidines investigated were 5-fluorouracil, 5-chlorouracil, 5-bromouracil, 5-iodouracil, 5-fluorocytosine, and 5-bromocytosine. The results for the halouracil a-cations are found to be in agreement with previous results found for the N-1 deprotonated cations identified in single crystals and acid glasses. The halocytosine a-cations gave evidence for a further reaction after production by C1,- attack. Both the 5-fluorocytosine and 5-fluorouracil a-cation radicals show large fluorine couplings (Azz= 150 G) and a single nitrogen coupling ( A , = 13 G) at low temperatures. However, in the case of 5-fluorocytosinewarming results in the conversion of the original a-cation to a new species whose fluorine splitting decreases dramatically to 115 G while two nitrogen splittings are found of nearly equal magnitude (Azz= 16 G). These results are shown to be excellent evidence th@tdeprotonation of the exocyclic nitrogen has taken place. Similar results are found with 5-bromocytosine. The reduction of the halogen splitting after deprotonation of the exocyclic nitrogen in 5-fluorocytosine and 5-bromocytosine is suggested to be a result of increased delocalization of the unpaired spin onto the exocyclic nitrogen. This is supported by INDO-MO calculations for the protonated and unprotonated structures.
Introduction Due to the enhanced effect of radiation on D N A containing 5-bromouracil in place of thymine, the radiation chemistry of 5-halouracils and their analogues has been of considerable int e r e ~ t . ' - ~ The enhancement has been proposed to be due to electron capture by 5-bromouracil in DNA followed by debromination to form the reactive 5-uracilyl radical.* Studies in nonaqueous system such as single crystals' and y-irradiated oriented bromouracil-substituted DNA9 have cast some doubt on the general applicability of this mechanism. It is therefore important that investigations of other possible reactive intermediates such as the halouracil a-cations be carried out. In a previous study Riederer and Hiittermann investigated the y-irradiation of several halouracils and derivatives in aqueous acidic glasses, 5 M HzS04 and 15 M H3POde2Evidence was found for formation of cation free radicals of these species. The cations were found either to deprotonate a t N-1 in the free bases or to add OH- to carbon 6 in the pyrimidine ring of the nucleosides. In recent work the oxidizing radical C12- has been shown to be an efficient oneelectron oxidizing agent for the production of D N A base cations at neutral to basic conditions. In studies of over 10 pyrimidines only the ?r- or a-cation radicals were observed after attack of C12produced by y-irradiation of 12 M LiCl glasses.lb12 In this work we have successfully analyzed the spectra of a number of halouracil and halocytosine a-cation radicals produced by the attack of C1, in y-irradiated 12 M LiCl glasses. The analysis included g, hyperfine, and nuclear quadrupole tensors (1) Hiittermann, J. Ultramicroscopy 1982, 10, 25. (2) Riederer, H., Hiittermann, J. J . Phys. Chem. 1982, 86, 3454. (3) Riederer, H.; Hiittermann, J.; Symons, M. C. R. J. Phys. Chem. 1981, 85, 2789. (4) Myers, L. S., Jr. In "Free Radicals in Biology"; Pryor, W. A,, Ed.; Academic Press: New York, 1980; Vol. IV, Chapter 3. p 95. (5) Sevilla, M. D.; Failor, R.; Zorman, G. J . Phys. Chem. 1974, 78, 696. (6) Farley, R. A.; Bernhard, W. A. Radiat. Res. 1975, 61, 47. (7) Kuwabara, M.; Lion, Y.; Riesz, P. Int. J . Radiat. Biol. 1981, 39, 491. (8) Zimbrick, J. D.; Ward, J. F.; Myers, L. S., Jr. Int. J. Radiat. Biol. 1969, 16, 525. (9) Graslund, A.; Rupprecht, A,; Kohnlein, W.: Huttermann. J. Radiat. Res. 1981, 88, 1. (10) Sevilla, M. D.; Suryanarayana, D.; Morehouse, K. M. J. Phys. Chem. 1981, 85, 1027. (11) Sevilla, M. D.; Swarts, S. J . Phys. Chem. 1982, 86, 1751. (12) Sevilla, M. D.; McGlashen, M. J . Phys. Chem. 1983, 87, 634.
__
0022-3654/84/2088-1601$01.50/0
for C1, Br, and I. Evidence for further reaction of the a-cations of 5-fluorocytosine and 5-bromocytosine involving the deprotonation of the exocyclic nitrogen is reported. Experimental Section The halouracils and halocytosines used in this study were obtained from Sigma Chemical Co. and were used without further purification. The LiCl used was the ultrapure grade obtained from Alpha Chemical Co. a-Cation radicals were produced by C12- attack in 12 M LiC1. This technique has been detailed in our previous work.'b12 Briefly, 12 M LiCl (DzO) containing about 1 mg/mL halopyrimidine, 5 mg/mL K,Fe(CN)63-, and sufficient NaOD to produce a 0.01 M solution is y-irradiated (80000 rd) at 77 K. The electrons produced are scavenged by Fe(CN)63-. The C12- produced by the radiation becomes mobile upon warming to 150 K and oxidizes the solute in a one-electron-transfer step. Computer simulations of the anisotropic spectra originating from 5-fluoropyrimidines were performed by use of the Lefebvre-Maruani program.I3 Simulations of the individual anisotropic ESR spectra for C1-, Br-, and I-containing D N A bases were performed by using a modified version of the MAGNSPEC program.14 The experiments performed in this work predominantly employed deuterated solutions. Work in water solutions gave similar results except that the resolution was poorer. This is due to two effects: increased dipolar coupling from the protons in the water over that from deuterons and broadening caused by unresolved proton hyperfine couplings a t exchangeable sites on the cation structures. For convenience the structures drawn in this paper show protons at these exchangeable sites. The reader should note that the spectra are for the cations in deuterated solutions. Results and Discussion 5-Halouracils. Irradiation of samples of 12 M LiCl at 77 K containing 5-halouracils (1 mg/mL) and 5 mg/mL K3Fe(CN), as as electron scavenger results predominantly in the C1; radical. (13) Lefebvre, R.; Maruani, J. J . Chem. Phys. 1965,42, 1480. (14) Mackey, J. A,; Kopp, M.; Tynon, E. C.; Yen, T. F. In "Electron Spin Resonance of Metal Complexes"; Yen, T. F., Ed.; Plenum Press: New York, 1960; p 33.
0 1984 American Chemical Society
1602 The Journal of Physical Chemistry, Vol. 88, No. 8, 1984 TABLE 1: ESR Spectral Parameters of the 5-Halouracil &ation 5-halouracil radical structure isotope A(halogcn), C
fluorouracil 1. x Xf
.l’ 2
P(halogen), MHz
fluorouracil I . X = 1:b
191:
191:
-15 -15 156
-16 -16 160
0 0
0 0 17 2.0060 2.0056 2.001 1
Radicals Produced by C12- Attack
.
x J’ 2
A(N,),G
X
Y z
g
14.5 2.0060 2.0060 2.0020
X
Y 2
Sevilla et al.
chlorouracil I , x = Cl= 3
5
~
1
bromouracil 1, X = Bra ‘I Brd -28 - 25 113 74 -49 - 25 0 0 13 2.0330 2.0270 2.0000
~
-4.9 -3.9 21 .o -12 6 6 0 0 12.5 2.0100 2.0090 2.001 8
bromouracil I, X = Brb Brd -37 - 25 124 68 -44 - 24 0
iodouracil I , x = Ia 1271e
-62 -49 115 -98 66 32
0 13 2.01 8 2.020 1.999
2.061 2.045 1.988
Previous investigation in 5.3 M H,SO, (ref 2). This work in slightly basic 12 M LiC1. Hyperfine (A) and quadrupole (P) tensors for the ”Cl isotope in its natural abundance were employed in the simulations with A(35C1)/A(37C1) = I .20 and P(35C1)/P(37C1) = I .28. Hyperfine and quadrupole tensors for the 79Br isotope in its natural abundance were also employed in the sirnulation with A(’l Br)/A(79Br) = 1.078 and P(*I Br)/P(”Br) = 0.85. e Parameters estimated from stick diagrams for selected orientations. The x axis is taken as parallel to the C-halogen bond. The y axis is perpendicular t o x and z , The z axis is perpendicular to the molecular plane. a
n
o
H
Figure 1. (A) ESR spectrum of the 5-fluorouracil ?r-cation radical (I, X = F) at 158 K produced by el2- attack on 5-fluorouracil 12 M LiCl (DzO). The central marker is at g = 2.0056. (B) Anisotropic computer simulation of the spectrum in A employing the parameters in Table I. A 5-G line width was employed for the central portion and a 7-G line width was employed for the wings of the spectrum.
Upon warming to the softening point of the glass (ca. 155 K) the C1, becomes mobile and attacks the solute as in reaction 1. The
H
rn 26.2G
‘i,/
I
J
Figure 2. (A) ESR spectrum of the 5-chlorouracil ?r-cation radical (I, X = CI) at 155 K produced by C l c attack in 12 M LiCl (D,O).The lower experimental tracing shows the upfield portion of the spectrum at 3 times the gain. (B) Anisotropic computer simulation of the spectrum in A employing the parameters in Table I.
X = I‘, C1, Br
spectra found a t 155 K for the 5-halouracils, 5-fluorouracil, 5chlorouracil, and 5-bromouracil, are shown in Figures lA, 2A, and 3A, respectively. The central marker in each spectrum is at g = 2.0056. The computer-simulated spectra are shown in Figures lB, 2B, and 3B. The g and A tensor principal values as well as the nuclear quadrupole couplings employed in the simulations are given in Table 1. The fit between the experimental and computer-simulated spectra is excellent in each case. The only significant deviation between experiment and the computer-simulated spectra is that there is an apparent decrease in intensity in the wings in the experimental spectra relative to the simulated spectra. This results from the fact that in the experimental spectra the line width is a function of the orientation of the radical to the external magnetic field. The line width is larger in the 11 orientation ( A z z )than in the I orientation (A,,, A?,,). This is a typical finding for anisotropic spectra where there are large differences in splittings in the 11 and Iorientations. Except as noted in the figure legends the computer simulations assume a single linewidth throughout. In the simulations of the 5-chlorouracil and 5-bromouracil r-cations nuclear quadrupole couplings ( P ) were employed. These values are relatively small for chlorine (-12, 6, and 6 MHz) but necesary for a reasonable fit to the experimental spectra. For
Figure 3. (A) ESR spectrum of the 5-bromouracil ?r-cation radical (I, X = Br) at 155 K produced by C12- attack in 12 M LiCl (D,O). The lower experimental tracing shows the upfield portion of the spectrum at 4 times the gain. (B) Anisotropic computer simulation of the spectrum in A employing the parameters in Table I. The 13-G nitrogen coupling is only partially resolved in the experimental spectrum and therefore the magnitude of this splitting should be considered approximate.
bromine the values are much larger (74, -49, and -25 MHz) and greatly affect the spectrum. Note in addition that tensors for both chlorine-37 and -35 as well as bromine-81 and -79 were employed in the simulations. This requires the summation of spectra d u e
5-Halouracil and 5-Halocytosine a-Cations
The Journal of Physical Chemistry, Vol. 88, No. 8, 1984 1603
to each isotope weighted in proportion to the natural abundance of the isotopes. In the case of 5-fluorouracil the large 156-G AZrFcoupling and the 14.5-G AZlNare clear evidence for the a-cation radical. Although either structure I or I1 with X = F can be considered
A
1
I1
possible structures, structure I is far more likely since the solutions were made slightly basic to aid the oxidation step. Indeed, the deprotonated ?r-cation has been observed previously in irradiated single crystals6J5and the reported values are in excellent agreement with those found here. In addition, the 5-fluorouracil and 5bromouracil a-cations both protonated and deprotonated at N-1 have been previously found in irradiated acidic glasses.2 In Table I we present values for hyperfine, g, and quadrupole tensors for the deprotonated radicals (I) for comparison. The distinction between the two radical types is made facile by the large change found in the parallel component of the halogen tensor (A,,) produced by a change in the state of protonation at N-1. For example, in acid glasses the A,, values for 5-fluorouracil and 5-bromouracil are 170 and 155 G when protonated and 160 and 124 G when deprotonated in the acid matrices. Our values (Table I) are clearly those for the deprotonated state, I. The value of 113 G found here for the A,, of the 5-bromouracil a-cation is 11 G smaller than that found in the acid matrix. This is considered a solvent effect. The electronic structure of the 5-bromouracil a-cation is m6re easily polarizable and thus a greater solvent perturbation than found for the less polarizable 5-fluorouracil a-cation is not unexpected? The deprotonated a-cation of 5-chlorouracil was not reported by Riederer and Huttermann in frozen glasses; however, data for the a-cation of 5-chlorodeoxyuridine were given.z The A , g, and P values found for this species are quite close to those found for the deprotonated 5-chlorouracil a-cation in this work and in single crystals.I6 For 5-chlorouracil, previous work in acidic glasses could find no clear division between the spectral parameters of the *-cation of its deprotonated derivative.z This is due to the fact that the powder-type simulations of the spectra do not reflect the small change in halogen interaction expected from predictions from INDO calculations.16 Thus, the spectrum for a a-cation in the nucleotide 5-chlorodeoxyuridine could well be reproduced by the data taken from the deprotonated species as found in single crystals.I6 The nearly same parameters also fit the spectrum obtained here from the deprotonated a-cation in the basic glass (Table I). In Table I we also report the analysis of the spectrum found for the 5-iodouracil a-cation. The values reported were determined from stick reconstructions employing stick diagrams at several selected orientations. As a consequence the parameters should be considered approximate. The a-spin density on the halogen in the halouracil a-cations can be estimated from the experimental A,, splittings from the relationship16 P V - w = Cf/2P)AZ,
withy= 0.67 for 19F,0.81 for 35Cl,0.81 for *lBr, and 1.0 for lZ7I. The factor 2P is the anisotropic coupling for unit spin density. We find that the r-electron spin density is 0.083 for F, 0.14 for C1, 0.19 for Br, and 0.21 for I, in their respective halouracil *-cation radicals. These results clearly show an increasing a-spin density on the halogen with an increase in the atomic number of (15) Close, D. M.; Farley, R. A.; Bernhard, W. A. Radiat. Res. 1978, 73, 212. (16) Oloff, H.; Hiittermann, J. J . Magn. Reson. 1980, 40, 415; 1977, 27, 197.
Figure 4. (A) ESR spectrum of the 5-fluorocytosine?r-cationradical (111, X = F) at 151 K produced by C1,- attack on 5-fluorocytosine in 12 M LiCl (DzO).(C) ESR spectrum of the deprotonated 5-fluorocytosine a-cation radical (IV, X = F) produced by warming the radical in A to 158 K. (B and D) Anisotropic computer simulations of the spectra in A and C employing the parameters in Table I1 and a 5-G line width. A 7-G line width was employed in the wings of B. The experimental spectrum in A shows significant line broadening in the wings; this may be due to unresolved nitrogen couplings. the halogen. This trend was reported previously for the protonated a-cations in glassesZ and the deprotonated cations in single crystals.6s'6 Oloff and Huttermann have performed INDO-MO calcuIations of the halogen spin density in these radicals and find the following a-spin densities on the halogens: 0.064 on F, 0.092 on C1, 0.094 on Br, 0.122 for I.16 Although these results are slightly lower than found here or in single crystals, the trend in the spin densities is correctly predicted. This trend can also be qualitatively understood by an examination of the ionization energies of the uracil fragment and the halogen. These ionization energies (IE) are a measure of the energy of the HOMO of the uracil fragment and the HOAO in the halogen. Coupling between the fragments is best when the interacting orbitals on each fragment are close in energy. Since the IE for uracil (an estimate of the uracil fragment) is 9.6 eV17 while those for F, C1, Br, and I atoms are 17.4, 12.9, 11.8, and 10.6 eV, respectively, it is readily apparent that the coupling between the fragments will increase with an increase in atomic number of the halogen. 5-Halocytosines. The *-cations of the 5-halocytosines have not been previously reported. Spectra for radicals produced from 5-fluorocytosineand 5-bromocytosine by Clz- attack in 12 M LiCl (DzO) glasses are given in Figures 4 and 5. The analysis of these spectra was tedious but straightforward. The parameters which best fit the experimental spectra are given in Table 11. The simulations based on these parameters are also shown in Figures 4 and 5. (17) Mc Glynn, S. P.; Dougherty, D.; Mathers, T.; Abdulner, S. In "Excited States in Organic Chemistry and Biochemistry"; Pullman, B., Goldblum, N., Eds.; Reidel: Boston, 1977; p 247.
1604 The Journal of Physical Chemistry, Vol. 88, No. 8, 1984
H
t+ 26,2G Figure 5. (A) ESR spectrum of the 5-bromouracil deprotonated r-cation (IV, X = Br) at 158 K produced by CIT attack in 12 M LiCl (DzO). (B) Anisotropic computer simulation of the spectrum in A employing the parameters in Table 11. The lower experimental tracing in A shows the high-field portion of the spectrum at 3 times the gain. Note that the structure which results from two equivalent nitrogens is reproduced in the computer simulation.
Sevilla et al. In previous work we have shown that the a-cations of 5methylcytosine, 5-hydroxymethylcytosine, and cytosine itself all undergo the analogous deprotonation reaction in 12 M LiCl glasses.l~lz*lsIn each of the above cases the radicals produced by this reaction showed two nitrogen couplings. The first was assigned to the nitrogen at position 1 by M O calculations. This coupling varied between 10 and 13 G. The magnitude of the second nitrogen splitting varied between 15 (5-hydroxymethylcytosine) and 24 (cytosine) G and was attributed to the exocyclic nitrogen on the basis of M O calculations.12 The 16-G coupling found here for 5-fluorocytosine is thus in the range of splittings found for exocyclic nitrogen couplings in other cytosine a-cations of structure IV. The second 16-G coupling found for 5-fluorocytosine (assigned to N-1) is a few gauss larger than found for the other cytosine cations.12 The cytosine cations all showed a decrease in the 5-position coupling as found in this work for 5-fluorocytosine. This decrease in 5-position coupling as well as the appearance of the exocyclic nitrogen coupling is predicted by simple HMO and INDO-MO calculations of the spin density distribution (discussed below). The effect of the deprotonation of the exocyclic nitrogen on the spin density distribution is most easily described by the following valence structures:
TABLE 11: ESR Spectral Parameters of the 5-Halocytosine
n-Cation Radicals Produced bv C L Attack 5-flUOrO5-halocytosine 5-flUOrOcytosine cytosine IV, X = F radical 111, X = F structure isotope 19F 19F A(halogen), G x -15 -13.4 Y. -15 -13.4 z 146 114.5 P(halogen), x MHz
Y
A(N,), G
x
o
Y
O
z x
13.8 c
S-bromocytosine IV, X = Br
Z
A(N,), G
Y
0 0
C
Br
X = F, Br
-19.8 -14.8 78.2 74 -49 -25
The deprotonation allows the third resonance form to contribute significantly. We note that there is another possible structure for this second radical found for 5-fluorocytosine, Le., structure V, X = F. This
0 0
16.0a
13b
0 0
0 0
16 .Oa 13b 2.0065 2.0063 2.0190 y 2.0065 2.0063 2.0170 z 2.0018 2.0020 2.0010 a,b These splittings were found to be equivalent to within the resolution of the experimental spectra (ca. 3 G). This splitting was found to be smaller than the line width. The nitrogen splitting from position 3 was also less than the line width. z
c
V
x
g
For 5-fluorocytosine, Clz- attack at 150 K results in an initial spectrum (Figure 4A) which quickly converts on slight warming to the more complex spectrum observed in Figure 4C. The parameters found from the analysis of Figure 4A (Table 11) are quite close to those found for the 5-fluorouracil a-cation. This spectrum is therefore clearly due to the N-1 deprotonated r-cation, structure 111, X = F. The spectrum found on warming gives evidence for a marked reduction in the fluorine coupling; Le., the AzzFvalue decreases from 146 to 114.5 G. In addition, the spectra show the appearance of a second nitrogen coupling of 16 G. This second coupling is found to be equal in magnitude to within the resolution inherent in the spectrum to the first nitrogen coupling, assigned to N-1. Simulations indicate that the two couplings could differ by no more than 2 G. The reduction of the fluorine coupling and the appearance of the second nitrogen coupling are expected for a radical of structure IV, X = F. This species is most likely produced by the following deprotonation reaction: y
2
-H+
0
I11
-$-+J IV
H
species is a tautomer of IV, X = F, and could be easily produced in these aqueous matrices. On the basis of ESR analysis alone it would be difficult to distinguish these two structures. In order to clarify which of the two structures is more likely, we have performed a number of INDO-MO calculations for structures 111-V with X = F using standard bond distances and angles.Ig The INDO results for V predicted an increase in the fluorine splitting over that found for 111. The results for IV predicted a decrease in the fluorine coupling as is found experimentally. In addition, the calculations show that for both IV and V nitrogens at positions 1 and 9 have the larger couplings while the nitrogen at position 3 is predicted to have a small (ca. 2 G) coupling. The INDO calculations give some support to our assignment of the second radical to a radical formed by deprotonation from the exocyclic nitrogen, IV, X = F. The results found for 5-bromocytosine (Figure 5, Table 11) did not give evidence for an intermediate radical as that found for 5-fluorocytosine. However, the analysis of the spectrum found at 155 K (Figure 5) shows coupling to two nitrogens and a substantially reduced bromine splitting from that found for 5bromouracil. From these results it is clear that the spectrum found is that associated with the a-cation radical deprotonated from the exocyclic nitrogen as well as N-1, Le., IV, X = Br. It is thus clear that both fluorocytosine and bromocytosine a-cations undergo reaction 2. Apparently this deprotonation step results in a radical (18) Sevilla, M. D.; Van Paemel, C.;Nichols, C. J . Phys. Chem. 1972, 76,
3571.
(19) Pople, J. A.; Beveridge, D. L. "Approximate Molecular Orbital Theory"; McGraw-Hill: New York, 1970.
1605
J . Phys. Chem. 1984, 88, 1605-1608 of greater stability. Since this reaction has been found in all substituted cytosines, it may be important in the radiation damage to DNA. Future experiments with cytosine-containing nucleosides will test this hypothesis. Acknowledgment. We thank the Office of Health and Environmental Research of the U S . Department of Energy (MDS)
and EURATOM-grant no. 21276-7 BIO D (J.H.) for support of this research. Registry No. I (X = F), 65987-90-8; I (X = Cl), 65105-58-0;I (X = Br), 65722-15-8;111 (X = F), 89043-28-7;1V (X = F), 89043-27-6; IV (X = Br), 89043-26-5; C12-, 12595-89-0; 5-fluorouracil, 51-21-8; 5-chlorouracil, 1820-81-1;5-bromouracil,51-20-7;5-iodouracil,696-07-1; 5-fluorocytosine, 2022-85-7; 5-bromocytosine, 2240-25-7.
Two Rotational Isomers of Methyl Thionitrite: Light-Induced, Reversible Isomerization in an Argon Matrix R. P. Muller and J. Robert Huber* Physikalisch-Chemisches Institut der Uniuersitat Zurich, CH-8057 Zurich, Switzerland (Received: August 16, 1983)
The infrared spectrum of methyl thionitrite, H3CSN0, in an argon matrix at 12 K shows that the molecule exists in a cis and trans conformation. With use of light between 485 and 590 nm the cis s trans isomerization processes can selectively be induced, permitting the formation of almost pure cis- or trans-methyl thionitrite. The vibrational frequencies of the two isomers were assigned, and AG0298(cis--ttrans)= 5.56 f 0.76 kJ mol-' was determined.
Introduction Nitrite molecules such as nitrous acid, HONO,' or methyl nitrite, H3CON0,2exist at room temperature in two rotameric conformations. These two forms-denoted as cis (syn) and trans (anti)-are clearly discernible from their IR spectra. Thionitrous acid, HSNO, which is prepared by photolyzing HNSO in an Ar matrix, was reported to show also cis and trans rotational isomer^.^ However, the difference between their IR spectra is small compared to that observed among the isomers of the nitrites. For methyl thionitrite, H3CSN0, in the gas phase, Philippe4 questioned the existence of two conformers since he had found no doubling in the IR absorption bands characteristic of a mixture of cis and trans isomers. H e suggested that H 3 C S N 0 at room temperature exists merely in the cis conformation, which is also the favorable geometry of H3CON0.Z Later Philippe and Ruths performed a normal-coordinate analysis and reassigned their data to the trans species. The trans-form geometry was also adopted by Byler and S u i 6 for a refined normal-coordinate analysis based on gas-phase measurements of several isotopes. Christensen et aL7 pursued the investigation of H,CSNO using I R (gas, liquid, solution), Raman (liquid), UV-vis (solution), and NMR (solution) techniques. While the optical spectra showed no evidence for a second isomer, such a species was inferred from a 'H N M R spectrum at -60 O C where a new weak peak appeared to the low field of the strong peak. By analogy with the thoroughly studied N M R spectrum of H3CON0, these peaks were assigned to the cis (strong peak) and trans (weak peak) forms. Recently Niki et aL8 explored spectroscopic and photochemical properties of H 3 C S N 0 in the gas phase. Recording the UV-vis absorption spectrum, these workers found beside the strong band at 340 nm two weak absorption maxima at 510 and 545 nm, (1) Hall, R. T.; Pimentel, G.C. J. Chem. Phys. 1963, 38, 1889 and references therein. (2) Felder, P.; Ha, T.-K.; Dwivedi, A. M.; Gunthard, Hs.H. Spectrochim. Acta, Part A 1981, 37A, 337 and references therein. (3) Tchir, P. 0.; Spratley, R. D. Can. J . Chem. 1975, 53, 2318. (4) Philippe, R. J. J. Mol. Spectrosc. 1961,6,442. Phillippe, R. J.; Moore, H. Spectrochim. Acta 1961, 17, 1004. (5) Philippe, R.J.; Ruth, J. M. J . Mol. Spectrosc. 1963, 1 1 , 331. (6) Byler, D.M.; Susi, H. J . Mol. Struct. 1981, 77, 2 5 . (7) Christensen, D. H.;Rastrup-Andersen, N.; Jones, D.; Klaboe, P.; Lippincott, E. R. Spectrochim. Acta, Part A 1968, 24A, 1581. (8) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. J . Phys. Chem. 1983, 87, 7.
0022-3654/84/2088- 1605$01.50/0
TABLE I: N=O Stretching Frequencies ;(cm-') of the Smallest Nitrites and Thionitrites
-2ra.s - "trans "cis "cis
-
a
Reference 1.
HONO in N,a
H,CONO in Arb
1684 1633
1665 1613 52
51
Reference 2.
HSNO in Arc
H,CSNO in Ard
1597 1548 1571 1527 26 21 Reference 3. This work.
confirming earlier results.' No assignment for the weak transitions was given. In accordance with the previous IR ~ o r k , their ~-~ FT-IR spectrum (resolution 1/ 16 cm-I) provided no evidence for the existence of a second isomer. Furthermore, photolyzing a gas-phase sample with light between 300 and 400 nm produced the radicals H3CS-and NO. This fragmentation reaction was followed by radical recombination, leaving the products H3CSSCH3 and NO. In the course of our study on light-induced conformational changes and photochemical transformations of nitroso compounds in low-temperature mat rice^,^ we also investigated thionitrites. In this paper we report the IR spectra of two conformers of H3CSN0, which, according to a very recent ab initio calculation,I0 possess the following geometry with the cis form predicted to be more stable (1-2 kJ mol-')
CIS
TRANS
(SYN)
(ANTI)
(9) Muller, R. P.; Murata, S.; Huber, J. R. Chem. Phys. 1982, 66, 237. Muller, R. P.:Russegger, P.; Huber, J. R. Ibid. 1982, 70, 281. Muller, R. P.; Huber, J. R. J . Phys. Chem. 1983, 87, 2460. (10) Bak, B.; Kristiansen, N. A,;Johanson, H. J . Mol. Struct. 1983, 100, 453.
0 1984 American Chemical Society
