J. Phys. Chem. 1996, 100, 16667-16673
16667
Reactions of Selenium in a Quartz Discharge Tube. Infrared Spectra and Density Functional Theory Calculations of New Selenium-Nitrogen and Selenium-Silicon Species in Solid Argon Lester Andrews,* Parviz Hassanzadeh, Dominick V. Lanzisera, and G. Dana Brabson Department of Chemistry, UniVersity of Virginia, CharlottesVille, Virginia 22901 ReceiVed: May 6, 1996; In Final Form: July 8, 1996X
New selenium-nitrogen species were produced by condensing the effluent from an argon/nitrogen/selenium microwave discharge onto a 12 K substrate. Nitrogen-15 and selenium-76 and -80 isotopic substitutions, photolysis, and annealing provided basis for identification of the new product absorptions in the infrared spectrum. A weak band at 951 cm-1 is assigned to SeN and the very strong 1019 cm-1 band to ν3 of NSe2 radical with a 146 ( 5° valence angle. A new 1570 cm-1 band in this system is due to the SeNO radical, and a 1771 cm-1 absorption, also made from Se2 and NO, is due to the (SeSe)(NO) complex. Higher-order SexNy (x > 1) species give rise to bands at 904, 736, 616, and 529 cm-1. The argon/selenium discharge in quartz has also produced the SiSe, SiSe2, and SeSiO species. Structure and frequency calculations using density functional theory support the matrix infrared identification of these new selenium molecular species. The most stable (NSe)2 structure is calculated to be the cis form analogous to (NO)2, but by contrast, (NSe)2 has a stronger NdN bond and Se-Se bonding.
Introduction Although nitrogen oxides are prevalent and important compounds in the chemistry of the elements, the stability of nitrogen chalconides decreases rapidly on going down the family in the periodic table.1 Sulfur nitride exists as the polymeric material polythiazyl (SN)x, and the evaporation products S2N2, S3N3, S4N2, and S4N4 have been characterized.2-5 The extensive binary cation chemistry of NS+, NS2+, and cyclic S3N2+ has been described.6-8 Conventional selenium-nitrogen compounds are limited to Se4N4, which is less stable than S4N4,9-12 and its reaction to form the cyclic Se3N2+ cation species has been recently studied.12 The SN radical has been studied extensively in the gas phase,13-15 and the analogous SeN radical has been examined by electronic and laser magnetic resonance spectroscopy.16-19 The N2S molecule has been examined by three groups,20-22 and the NS2 radical has been characterized by matrix infrared spectroscopy22 and ab initio calculations.23,24 Since no data could be found for any SeN molecular species other than the diatomic molecule, microwave discharge of selenium/nitrogen mixtures in argon carrier gas was employed to prepare and trap the most stable binary nitrogen/selenium molecular species. A preliminary account of the formation of NSe2 radical has appeared.25 Although earlier work with microwave discharge of selenium vapor has produced infrared absorptions for Se3 and Se4 clusters,26 it must be recognized that the silica tube is a source of both silicon and oxygen. In similar work with sulfur, reaction in the silica discharge tube produced SiS at 739.1 cm-1 and SiS2 at 917.9 cm-1 and supported the earlier identification27 of these molecules through the use of isotopic sulfur.28 Since isotopic selenium samples can be used in this work, identification of the analogous SiSe and SiSe2 molecules will be sought. The spectra of selenium oxides will be reported elsewhere.29 Silicon and selenium form a glassy solid of formula SiSe2 with medium-range order,30 but there is no information on the SiSe2 molecular species. Following SiO2 and SiS2,27 the SiSe2 X
Abstract published in AdVance ACS Abstracts, September 15, 1996.
S0022-3654(96)01288-9 CCC: $12.00
molecule is expected to be linear and strongly bound. The diatomic molecule SiSe has been studied in the gas phase and has a vibrational fundamental of 571.6 cm-1.31 It has been pointed out that the difference in force constants between the triple and double bonds for CO and CO2 is diminished in SiO and SiO2, and in the sulfides,27 a trend expected in the selenides as well. Experimental Section The apparatus for microwave discharge matrix isolation studies has been described previously.22,26,28,32 A closed-cycle refrigerator (CTI Cryogenics, Model 22) and thermocouple indicator/controller cooled and monitored the temperature of the CsI window. FTIR spectra were recorded on a Nicolet 60SXR spectrometer at 0.5 cm-1 resolution (KBr beam splitter) and (0.1 cm-1 frequency accuracy. Natural selenium (Alfa, 99.9%), enriched isotopic samples (selenium-76, >99%, and selenium-80, >92%, Kurchatov Institute, Moscow), nitrogen gas (Matheson, zero gas), and nitrogen-15-enriched gas (Cambridge Isotope Laboratories) were used as received. Typically, 2% N2 in Ar mixtures were employed. The quartz tubes used for seeding selenium vapor into an argon/nitrogen discharge were similar to the tubes described previously with a 1 mm orifice. The discharge apparatus was operated as reported by Brabson and Andrews:26 selenium powder was placed in the reservoir side tube and heated to near 230 ( 10 °C to provide a light sky blue color from Se2 emission in the top of the reservoir tube. The nominal 100 W microwave discharge operated at 80% power, and the gas flow rate was adjusted so that the glow protruded the tube orifice. It was important to keep the cold window at 12 ( 1 K; temperatures above 16 K were not capable of trapping the new product species in good yield. The discharged gas stream condensed at 12 ( 1 K for 10-12 h periods at 2 mmol/h, and FTIR spectra were recorded before and after annealing and photolysis as described for analogous sulfur experiments.22 © 1996 American Chemical Society
16668 J. Phys. Chem., Vol. 100, No. 41, 1996 Results Matrix infrared experiments were performed with three different selenium isotopic samples: natural selenium, selenium76, and selenium-80. DFT structure and frequency calculations were performed for possible product molecules. Natural Selenium and Nitrogen. Eight experiments employed natural isotopic selenium. A sharp CSe2 band at 1298.1 cm-1, a weak SiO band at 1225.6 cm-1, and a very sharp 513.65 ( 0.05 cm-1 Se2 electronic band were observed in all experiments.26,32-34 The new product absorptions are dominated by a very strong 1019 cm-1 absorption and include 1771, 1570, 1456, 1251, 952, 904, 736, 751, 572, 616, and 529 cm-1 product bands. Photolysis decreased the 1456, 1251, 1019, 736, and 616 cm-1 bands (by 10-30%) with little effect on the other absorptions. Annealing to 25 K decreased the 1251 cm-1 band and the NO band by 40%, decreased the 1570 cm-1 band by 10%, increased the 1771 cm-1 band by 20%, and produced new features at 1824 and 1688 cm-1. Annealing to 30 K markedly decreased the 1251 cm-1 band, decreased the 1771 and 1570 cm-1 bands, broadened the 1019 cm-1 band, increased the 736 and 616 cm-1 bands, increased and resolved the 529 cm-1 band into sharp 528.5 and 523.8 cm-1 components, and decreased the 513.65 cm-1 Se2 band. Annealing also provided green and blue emissions characteristic of N and Se atom recombinations. All of the bands (except 513.65 cm-1) showed nitrogen-15 shifts. The 736.2 cm-1 band with 14N2 shifted to 714.8 cm-1 with 15N2 leaving behind a weak 736.6 cm-1 discharge impurity C2H2 band. Experiments with mixed 14N2/15N2 in the discharge gave a superposition of the two pure isotopic spectra without any new intermediate features. Two experiments were run on a PE 983 grating instrument to extend the wavenumber range. In addition to the bands reported above, absorptions were observed at 370 and 348 cm-1 with 3 times the absorbance as 951, 616, 528, and 513 cm-1 bands. The former bands have been assigned to Se4 species.26 Selenium and NO. Three experiments were done with natural selenium and NO. The first codeposited selenium/argon discharge products with NO in argon. Similar products were observed, but the 1771 and 1570 cm-1 bands were markedly enhanced, the 1019 cm-1 band was still prominent, and other absorptions were weak. The second codeposited Se2 from the superheater source26 with NO/Ar, and only the 1771 cm-1 band was observed. The third codeposited selenium/16,18O2/N2 discharge products and strong doublets were observed at 1771.4, 1725.0 cm-1 and at 1570.0, 1531.8 cm-1, along with selenium oxides, which will be discussed in a separate report.29 Selenium-76. In an experiment with enriched selenium-76 and 14N2, all of the bands were slightly higher in frequency than the natural isotopic measurements, except for the 513.65 cm-1 Se2 band. One experiment combined selenium-76 and mixed 14N /15N in the discharge, and the frequencies are given in Table 2 2 1. Sharp new bands were observed at 912.8, 886.7, 753.7, and 575.4 cm-1 that showed no nitrogen isotopic shift. Sample annealing helped track the bands for each isotopic component. Again, the product bands exhibited mixed nitrogen isotopic doublets. Infrared spectra for these experiments are given in Figures 1-3, traces a and b. Selenium-80. In two analogous experiments with enriched selenium-80, the bands were slightly lower in frequency than natural isotopic frequencies, except for the 513.65 cm-1 Se2 band. Figures 2 and 3 contrast isotopic selenium spectra in the lower regions of interest here. Analogous sharp, new bands were observed at 908.9, 883.0, 750.8, and 571.7 cm-1, which did not shift with isotopic nitrogen. In the upper region, the
Andrews et al. TABLE 1: Absorptions (cm-1) Observed on Condensation of Argon/Nitrogen/Selenium Stream Subjected to Microwave Discharge 76-14
80-14
76-15
80-15
ident
1872.5 1863 1776 sh 1771.4 1570.2 1463.5 1300.1 1254.2 1253.3 1253.3 1248.2 1021.2 955.0 947.9 912.8 906.8 886.7 753.7 737.5 617.2 575.4 529 513.65
1872.5 1863 1776 sh 1771.4 1570.0 1455.2 1297.8 1254.2 1253.3 1250.7 1245.5 1019.0 951.4 944.4 908.9 902.9 883.0 750.8 735.1 614.7 571.7 527 513.65
1839.6
1839.6
1745 sh 1740.5 1542.2 1426.0 1300.1
1745 sh 1740.5 1542.0 1417.0 1297.8
1215.0 1210.0 990.3 928.1 921.0 912.8 881.2 886.7 753.7 716.9 599.3 575.4 514 513.65
1212.4 1207.4 988.0 924.3 917.5 908.9 877.8 883.0 750.8 714.4 597.3 561.7 512 513.65
NO (NO)2 (NO)2 (Sex)(NO) SeNO NSe2(ν1 + ν3) CSe2 SeSiO SeSiO NSe+2(ν3) NSe+2 site NSe2(ν3) SeN SeN site SeO NSeNSe SeSeO SiSe2 cis-SeSeNSe SeSeN SiSe trans-SeSeSeN Se2
1771.4 cm-1 band showed no selenium isotopic shift while the 1570.2 cm-1 band shifted to 1570.0 cm-1. Calculations. Density functional theory (DFT) calculations were performed on possible product molecules, with a fine integration grid, using the Gaussian 94 program package.35 The B3LYP functional36 was employed with 6-311G* basis sets for each atom.37 Geometry optimizations used redundant internal coordinates and converged via the Berny optimization algorithm.35,38 From these geometries, the vibrational frequencies were calculated, and Tables 2-5 and Figure 4 present the results of these calculations. Discussion The new binary selenium nitride species produced here will be characterized by selenium and nitrogen isotopic shifts. The present selenium seeded discharge was a lighter blue than observed in previous work with Se3 and Se4 clusters; hence, the major new species produced here should have fewer selenium atoms. Mixed nitrogen isotopic experiments revealed doublet product absorptions in all cases, indicating single N product species. SeN. The 952 cm-1 band with natural selenium is in the region expected for the SeN diatomic molecule because the vibrational fundamental for 76Se14N in the gas phase is 948.198 cm-1. The 952 cm-1 band is resolved into 955.0 and 947.9 cm-1 bands with selenium-76. The 76-14/76-15 isotopic ratios are 1.0290 and 1.0292, just under the harmonic value (1.0293) expected for cubic anharmonicity. The 76-14/8014 ratios 1.003 78 and 1.003 71 are just below the 1.003 87 gas phase value and the 1.003 92 harmonic value. One matrix value is blue-shifted 6.8 cm-1 from the gas phase value, and the other is red-shifted just 0.3 cm-1. It is possible that the blue-shifted matrix site interacts more strongly due to complete surrounding by argon, and the red-shifted site is due to SeN on an argon crystallite surface. The similar SN molecule exhibits two argon matrix site absorptions at 1209.4 and 1203.1 cm-1, one blueshifted 5.3 cm-1 and the other red-shifted 1.0 cm-1 from the gas phase value.22 Density functional theory with the B3LYP functional was employed to calculate structures and frequencies for the new selenium species prepared here. These calculations predict SeN
Reactions of Selenium in a Quartz Discharge Tube
J. Phys. Chem., Vol. 100, No. 41, 1996 16669
Figure 1. Infrared spectra in the 1900-1500 cm-1 region for products of Ar/N2/Sex microwave discharge condensed at 12 ( 1 K for 10 h. (a) 76Se + 14N2 and (b) 76Se + 14N2 + 15N2. Many weak bands are due to incomplete purge of water vapor.
Figure 2. Infrared spectra in the 1270-910 cm-1 region for products of Ar/N2/Se microwave discharge condensed at 12K for 10 h: (a) 76Se + 14N , (b) 76Se + 14N /15N , (c) 80Se + 14N , and (d) 80Se + 14N /15N . 2 2 2 2 2 2
at 978.4 cm-1 (Table 2), 27 cm-1 (or 2.8%) above the major matrix site. The predicted bond length (1.656 Å) is in excellent agreement with the gas phase value (1.65 Å).16 NSe2. The very strong 1019 cm-1 band dominates the product spectrum in natural isotopic selenium experiments. With 76Se the band was observed at 1021.2 cm-1, and with 80Se the band shifted to 1019.0 cm-1. Following assignment of the strongest product band in the sulfur-nitrogen discharge system to bent NS2, where mixed sulfur isotopic spectra were resolved into a 1/2/1 triplet, the 1019.2 cm-1 band is assigned to ν3 of NSe2. Unfortunately, the small selenium isotopic shift prevented
the resolution of a mixed selenium isotopic multiplet. However, the Se and N isotopic shifts define a bent SeNSe molecule.25 Isotopic ν3 frequencies have been used to determine the valence angles of bent C2V molecules like SO2, S3, and NS2.22,32,39 Terminal isotopic substitution gives an upper limit to the valence angle, central isotopic substitution gives a lower limit, and the average of these angles provides a measure of the valence angle owing to the averaging of isotopic anharmonic differences.39 The selenium 76/80 isotopic ratios for 14N and 15N (1.002 16 and 1.002 33, respectively) define 165° and 159° upper limits, and the nitrogen 14/15 isotopic ratios for 76Se and
16670 J. Phys. Chem., Vol. 100, No. 41, 1996
Andrews et al.
Figure 3. Infrared spectra in the 760-500 cm-1 region for matrix samples presented in Figure 1.
TABLE 2: Frequency (cm-1), Intensity (km/mol), and Structure Calculations for Triatomic Selenium-Nitrogen Compounds Using B3LYP Density Functional Theory
molecule
frequencies (intensity)
bond lengths (Å)
SeN NNSea NSeNa (N2)Sea SeNSec SeSeNc (SeNSe)+ SeNOd SeNOe NSeOd
978.4 (7) 2186.9(359), 510.9(15), 403.4(0) 1057.1(16), 924.1(0), 166.1(7) 903.6(3), 760.6(4), 305.1(1) 1040.4(298), 445.5(6), 205.7(4) 643.4(29), 318.0(3), 155.6(6) 1264.9(287), 399.7(0), 314.5(28) 1691.9(572), 570.5(37), 419.8(1) 1607.2(481), 555.2(28), 408.2(1) 872.5(40), 721.6(3), 244.7(25)
1.656 1.121,b 1.773b 1.628 1.717 1.715 2.254, 1.743 1.653 1.787, 1.177 1.803, 1.192 1.717, 1.647
bond angles (deg) 180 180 71.3 149.7 108.8 180 138.1 137.3 113.9
a Relative energy of isomers is 0, +132, and +141 kcal/mol going down the column. b Bond lengths given for bonds in molecule, left to right, respectively. c Relative energy of isomers is 0 and +20 kcal/ mol. d Relative energy of isomers is 0 and +20 kcal/mol. e Calculated with BP functional.
80Se (1.031 20 and 1.031 38, respectively) define lower limits of 130° and 129°. The average value 146° should have an uncertainty on the order of 5°. This 146 ( 5° value is within the 153 ( 5° angle calculated for NS2 from isotopic ν3 frequencies,22 the 150-155° range from high level ab initio calculations,23 and the 149.7° value predicted here for NSe2 by DFT calculations. The weaker band at 1455 cm-1 tracks with the 1019 cm-1 band on photolysis and annealing and shows appropriate isotopic shifts for assignment to the ν1 + ν3 combination band of NSe2. Subtracting the ν3 values for each isotopic molecule from the combination band leaves ν1 values of 442.3, 435.7, 436.2, and 429.5 cm-1 for 14N76Se2, 15N76Se2, 14N80Se2, and 15N80Se2, respectively. These frequencies define nitrogen isotopic shifts of 6.6 and 6.7 cm-1 and selenium isotopic shifts of 6.1 and 6.2 cm-1, values that are appropriate for the ν1 mode of bent NSe2. For a 150° valence angle and neglecting any interaction with the ν2 bending mode expected near 200 cm-1, the nitrogen isotopic shift is calculated to be 6.8 cm-1 for 76Se, and the selenium isotopic shift is calculated to be 6.7 cm-1 for 14N.
These values agree very well with isotopic shifts deduced from the ν1 values calculated above. DFT calculations predict ν3 and ν1 frequencies at 1040.4 and 445.5 cm-1 for SeNSe radical. These values are in excellent agreement with the observed 1019.0 cm-1 ν3 fundamental and the 436.2 cm-1 ν1 fundamental deduced from the ν1 + ν3 combination band (2.1% high for each) and confirm the assignment of these bands. SeNSe+. The 1253.3 cm-1 band decreases by 20-30% on photolysis and is virtually destroyed by association and reaction with other molecules present on annealing. This behavior is analogous to the 1499.7 cm-1 band in the sulfur-nitrogen system. The 1499.7 cm-1 band is characterized by 34S and 15N isotopic shifts, a triplet with mixed 32,34S, and a doublet with mixed 14,15N to be an antisymmetric NS2 vibration similar to that for the NS2 radical. The photolysis and annealing behavior show that the 1499.7 cm-1 band is due to an even more reactive species than the NS2 radical. The observed isotopic ratios agree very well with those calculated for the ν3 mode of a linear S-N-S species. The 1499.7 cm-1 argon matrix band must be reassigned to isolated SNS+ in solid argon. This reassignment is confirmed by the ν3 assignments to (NS2+) in (AsF6-) and (SbCl6-) salts at 1494 and 1498 cm-1.40 The cation can be produced by photoionization of the radical with vacuumultraviolet radiation in the argon discharge.41 The 1253.3 cm-1 band shows a doublet with 14N2/15N2 mixtures in the discharge and the isotopic ratios agree well with those calculated for ν3 of a linear Se-N-Se species.25 (The spectrum in Figure 1 of the earlier communication25 gives a better display of this band.) Hence, the 1253.3 cm-1 band is assigned to ν3 of linear Se-N-Se+. In support of this identification of NSe2+, the isoelectronic linear CSe2 molecule absorbs at 1300 cm-1 in solid argon, and DFT calculations predict a linear (SeNSe)+ cation with ν3 at 1264.9 cm-1, only 0.9% higher than the observed value, as compared in Table 2. SeNSeN. The 906.8 and 902.9 cm-1 absorptions with 76Se and 80Se, respectively, show nitrogen 14/15 isotopic ratios of 1.029 05 and 1.028 95 and a selenium isotopic 76/80 ratio of 1.004 32. The nitrogen ratios are slightly lower than the SeN
Reactions of Selenium in a Quartz Discharge Tube
J. Phys. Chem., Vol. 100, No. 41, 1996 16671
TABLE 3: Frequency (cm-1), Intensity (km/mol), and Structure Calculations for Tetraatomic Selenium-Nitrogen Compounds Using B3LYP Density Functional Theory molecule a
SeNNSe (singlet cis) Se2N2a (singlet rhombus) SeNSeNa (triplet trans) NSeSeNa (triplet cis) SeNSeNa (singlet cis) SeSeNSec (doublet cis) SeSeSeNc (doublet cis) SeSeSeNc (doublet trans)
frequencies (intensity)
bond length (Å)
1542.1 (159), 710.0 (35), 457.5 (18), 403.9 (0), 262.1 (4), 224.8 (16) 785.1 (0), 743.2 (0), 604.1 (20), 545.7 (6), 362.5 (0), 339.7 (25) 971.0 (84), 829.6 (19), 357.1 (53), 200.6 (4), 118.0 (0.7), 74.3 (0) 900.4 (14), 757.6 (403), 171.8 (1), 143.8 (0), 107.5 (0), 88.4 (2) 1009.5 (53), 881.4 (40), 362.4 (3), 180.0 (3), 110.4 (3), 68.8 (17) 741.9 (61), 428.4 (30), 316.3 (2), 245.8 (0), 206.6 (7), 72.2 (0) 834.3 (48), 658.2 (36), 288.6 (1), 233.6 (10), 212.8 (3), 86.8 (1) 606.0 (124), 328.9 (21), 213.4 (6), 103.7 (0), 71.2 (2), 44.6 (4)
1.968,b
1.219b
bond angles (deg) 108.3b
1.822
N-Se-N ) 88.9
1.719, 2.002, 1.626
126.6, 112.8
1.668, 2.755
116.6
1.680, 2.168, 1.631
112.5, 109.3
1.739, 1.857, 2.249
124.7, 113.8
2.685, 2.463, 1.720
67.8, 94.6
2.256, 2.497, 1.710
108.0, 109.0
a Relative energy of isomers is 0, +33, +63, +69, and +70 kcal/mol. b Bond lengths and angles given for bonds in the molecule, left to right, respectively. c Relative energy of isomers is 0, +1, and +15 kcal/mol.
TABLE 4: Calculated Bond Lengths (Å) for cis-XNNX (X ) O, Si, Se) Species Using B3LYP Density Functional Theory X
rN-N
rX-N
rX-Xa
O S Se
2.014 1.250 1.219
1.149 1.761 1.968
2.404 2.164 2.453
a
In contrast, the diatomic O2, S2, and Se2 bond lengths are 1.208, 1.889, and 2.166 Å. See: Huber, K. P.; Herzberg, G. Constants of Diatomic Molecules, Van Nostrand Reinhold: New York, 1979.
TABLE 5: Frequency (cm-1), Intensity (km/mol), and Structure Calculations for Silicon, Selenium, and Oxygen Compounds Using B3LYP Density Functional Theory molecule
frequencies (intensity)
bond lengths (Å)
SiO SeO SiSe SeSiO SeSiSe
1248.6 (40) 891.4 (5) 572.0 (28) 1277.9 (111), 470.8 (6), 218.7 (2 × 29) 738.4 (123), 288.8 (0), 140.7 (2 × 2)
1.520 1.663 2.080 2.061, 1.519 2.071
values and the selenium ratios slightly higher. A 1167.4 cm-1 band in S/N experiments behaved similarly and was tentatively assigned to SN dimer, owing to its appearance just below SN at 1209 cm-1. The analogous assignment follows for the 902.9 cm-1 band. However, the lack of a triplet in mixed 14/15 nitrogen experiments suggests a preference for the alternating SeNSeN arrangement with two inequiValent N atoms. DFT calculations were done for five possible Se2N2 structures in singlet and triplet states. The global minimum-energy species is singlet cis-SeNNSe analogous to (NO)2 in structure, but the SeNNSe species has a much stronger NdN bond and the Se atoms bond with each other forming a cyclic C2V molecule. Calculated frequencies and bond lengths for the Se2N2 species are given in Table 3. A similar structure has been found to be the global minimum in calculations on S2N2 structures.42 DFT calculations on the analogous SNNS molecule yield a structure similar to that of previous RHF calculations42 and that of SeNNSe. Table 4 presents comparisons of DFT calculations on the ONNO, SNNS, and SeNNS molecules. Despite the relative stability of the SeNNSe, this species is not observed here. The next lowest energy structure is singlet rhombus Se2N2 (almost square), 33 kcal/mol higher than the global minimum. Although metal complexes of Se2N2 have recently been reported,43 no evidence of such a species with two equiValent
Figure 4. Structures for the major selenium species calculated by density functional theory.
N atoms was found here. This corresponds to the well-known alternating S2N2 ring calculated 20 kcal/mol higher than the pairwise ring.42 The next structure is triplet trans-SeNSeN, 63 kcal/mol higher energy than the global minimum, which has two inequiValent N atoms. The calculations predict one strong band at 971 cm-1, which is 7.5% higher than the observed 903 cm-1 band. The calculated 14/15 ratio, 1.0296, is in excellent agreement with the observed 1.0286 ratio. Furthermore, the mixed 14-15 isotopic calculations show splittings too small to be resolved, in accord with the observed spectrum. Observation of the higher energy SeNSeN species indicates that this dimer is formed from the SeNSe + N addition reaction. Therefore, dynamics governs the formation of Se2N2 species more than energetics. Although the cis singlet is only 70 kcal/mol higher than the global minimum, the two strong calculated frequencies (Table 3) are not appropriate. Finally, the triplet cis-NSeSeN species is 69 kcal/mol higher than the global minimum, has equivalent N atoms, and is not observed here. SexN Species. The 736 and 616 cm-1 absorptions (natural isotopic values) have common behavior: the bands decrease on photolysis and grow on annealing although the former appears to follow such changes more than the latter, and the bands are probably not due to the same species. Both bands are much stronger than anything in the analogous region in S/N experiments. Both bands exhibit slightly smaller 76/80 ratios than SeN, but the 616 cm-1 band shows slightly larger and the 736 cm-1 band slightly smaller 14/15 ratios than SeN. Both bands exhibit mixed nitrogen isotopic doublets denoting a single N atom vibration, and they occur in the region of Se-N single bond stretching frequencies for the Se4N4 compound (770, 621,
16672 J. Phys. Chem., Vol. 100, No. 41, 1996 and 570 cm-1).11,12 The bands are assigned to Se-N vibrations in open SexN species. DFT calculations were done for four doublet Se3N isomers. The lowest energy structure was cis-SeSeNSe with the cisSeSeSeN arrangement only 1 kcal/mol higher. However, the calculated frequencies were considerably different; the former exhibited one strong band, 741.9 cm-1, with 14/15 ratio 1.0300, and the latter, two strong bands at 834.3 and 658.2 cm-1. The observed 735.1 cm-1 band (15/15 ratio 1.0290) is assigned to cis-SeSeNSe. Calculations on the SeSeN isomer predict a very strong 643.4 cm-1 band with 14/15 ratio 1.0299; the observed 614.7 cm-1 band with 14/15 ratio 1.0291 is accordingly assigned to the bent SeSeN radical, the higher energy isomer of the major SeNSe radical product. The 528.5 cm-1 band and a satellite at 523.8 cm-1 increase markedly on annealing. The 15N counterpart is difficult to measure, owing to overlap with the 513.6 cm-1 Se2 electronic transition, but the band shows a 14/15 ratio of 1.029 comparable to that of SeN. DFT calculations on trans-SeSeSeN revealed an energy 14 kcal/mol higher than cis-SeSeSeN but only a single strong band at 606.0 cm-1. The calculated 14/15 isotopic ratio, 1.030 78, is in reasonable agreement with experiment. We therefore attribute the 528.5 cm-1 band to trans-SeSeSeN. Se2. The sharp 513.65 ( 0.05 cm-1 band (fwhm ) 0.7 cm-1) shows no 76Se-80Se-nSe displacement. It has been previously assigned25 to the absorption between the fine structure components X1g r XOg+ for the Se2 electronic ground state. Its observation here with no isotopic shift for pure 76Se2 and 80Se2 confirms that assignment. Selenium-NO Species. Weak bands were observed at 1872.5 cm-1 for NO and 1863, 1776 cm-1 for (NO)2.44,45 Two other strong bands in the N-O stretching region, 1771 and 1570 cm-1, are appropriate for ternary species. The SNO molecule has been characterized by strong Fermi resonance bands at 1597 cm-1 (2νN-S) and 1523 cm-1 (νN-O).34 The strong band at 1570.0 cm-1 shows a small (0.2 cm-1) selenium 76-80 shift and the appropriate 14/15 ratio (1.018 16) and 16/18 ratio (1.024 94) for a N-O stretching vibration (diatomic 14/15 ratio 1.017 88 and 16/18 ratio 1.026 61). The band forms sharp doublets in mixed 14N2/15N2 and mixed 16,18O2 isotopic experiments, indicating a vibration of single N and O atoms. Accordingly, the 1570 cm-1 band is assigned to the N-O stretching mode of the bent SeNO molecule analogous to NO2, NS2, and SNO.22,45,46 The small (0.2 cm-1) selenium isotopic shift indicates some coupling between Se-N and N-O stretching motions as does the fact that the 14/15 ratio is slightly higher and the 16/18 ratio slightly lower than the diatomic N-O ratios. Note that Fermi resonance is absent in SeNO as the strong 1570 cm-1 N-O stretching band is out of reach of the overtone of the weaker Se-N stretching mode calculated at 570 cm-1 (Table 2). DFT calculations for SeNO predict a very strong 1691.9 cm-1 band which is too high by 122 cm-1. Since previous DFT calculations with the BP functional for SNO are in better agreement with experiment, similar calculations were performed for SeNO. The strongest band is predicted at 1607.2 cm-1, in much better agreement. The NSeO isomer was calculated to be 20 kcal/mol higher in energy, with a strong Se-N stretching mode at 872.5 cm-1 and a much weaker Se-O stretching mode at 721.6 cm-1. No evidence was found for this species. On the other hand, the 1771.4 cm-1 band exhibits no selenium isotopic shift and a 14/15 ratio (1.017 75) and 16/18 ratio (1.026 90) much nearer the diatomic N-O ratios. The appearance of this band as isotopic doublets in 14N2/15N2 and 16,18O2 experiments further identifies the vibration of single nitrogen
Andrews et al. and oxygen atoms. An analogous 1772.5 cm-1 band has been observed in sulfur work and assigned to the SS-NO complex.46 The 1771.4 cm-1 band is due to a selenium cluster-NO complex SeSe-NO. This complex is stable, like SS-NO, and not as reactive as OO-NO. The Se2 + NO experiment gave no evidence for SeNO, unlike 18O2 + NO studies, which gave 18ON16O.46 The bands that grow on annealing at 1824 and 1688 cm-1 are probably due to N2O3 analogs45-47 with selenium replacing 1 or 2 oxygens. Silicon Selenides. The weak 572 cm-1 band is in excellent agreement with the gas phase SiSe fundamental,31 571.6 cm-1, and the band is assigned accordingly. The 76SeSi and 80SeSi fundamentals 575.4 and 571.7 cm-1 give the ratio 1.006 47, which is slightly lower than the harmonic diatomic ratio 1.006 82, as expected for anharmonicity. The SiSe fundamental calculated by DFT is in excellent agreement ((1 cm-1)! The linear SSiS molecule exhibits a strong ν3 fundamental at 917.9 cm-1, some 24% above the SiS diatomic fundamental at 739.1 cm-1, both in solid argon. This predicts ν3 of SeSiSe above 700 cm-1, and the 751 cm-1 band is appropriate. The 76Se Si and 80Se Si isotopic bands at 753.7 and 750.8 cm-1 give 2 2 a 1.003 86 ratio, which is slightly lower than the harmonic value calculated for the ν3 mode of the linear molecule, 1.003 92. This agreement in calculated and observed isotopic ratio supports the identification of the linear SeSiSe molecule following the examples of SSiS and OSiO. Finally, DFT predicts the strong ν3 fundamental at 738 cm-1, just 1.7% lower than the observed value (Table 5). Schno¨ckel and Ko¨ppe point out that bonding in the diatomic and triatomic group 4-6 molecules converges for the heavier members of this series.27 Accordingly, if the Se-Si stretching force constant (Frr) for SeSi2 is approximated by the 4.043 mdyn/Å diatomic value, and the stretch-stretch interaction force constant is set to zero, ν3 of SiSe2 is predicted at 762 cm-1. A finite positive Frr will decrease this frequency. Accordingly, the 750.8 cm-1 band is assigned to SiSe2. The 1253.3 cm-1 band appears to be unusually strong in Figure 1a using 76Se. Notice that a 1254.2, 1253.3 cm-1 doublet remains when the (NSeN)+ product is shifted down with 80Se and 15N. This 1254.2, 1253.3 cm-1 band shows no selenium isotopic shift, and it falls in the range expected for Si-O vibrations. The diatomic SiO fundamental is 1225.7 cm-1 in solid argon, and in the SSiO molecule, Fermi resonance divides this fundamental into bands at 1265.4 and 1289.9 cm-1.48 The present 1254.2, 1253.3 cm-1 band is assigned to the Si-O fundamental in SeSiO. Note that, like SeNO and SNO, the Fermi resonance found in SSiO is removed in SeSiO. Finally, the Si-O mode in SeSiO is predicted by DFT at 1277.9 cm-1, 24 cm-1 higher than experiment, in agreement with the DFT prediction of SiO at 1248.6 cm-1, 22 cm-1 higher than experiment. Other Selenium Species. The sharp 1300 cm-1 band is near the ν3 fundamental of CSe2 assigned as 1303 cm-1 in the gas phase.34 The sharp matrix bands for pure 76Se and 80Se determine the 76/80 isotopic ratio 1300.1/1297.8 ) 1.001 772. The calculated harmonic 76/80 ratio for ν3 of the linear SeCSe molecule, 1.001 840, is in excellent agreement and supports this identification of CSe2. The sharp 912.8 and 886.7 cm-1 bands for 76Se shift to 908.9 and 883.0 cm-1 for 80Se, which define 912.8/908.9 ) 1.004 29 and 886.7/883.0 ) 1.004 19 ratios. This, and the absence of nitrogen isotopic shift, is appropriate for Se-O vibrations. In a separate study of Se/O reactions, the bands shifted from 909.1 and 883.1 cm-1 with natural isotopes to 866.3 and 841.2 cm-1
Reactions of Selenium in a Quartz Discharge Tube on oxygen-18 substitution, which characterizes Se-O vibrations in selenium oxide species.29 Finally, the 2000 cm-1 region was searched for evidence of N2Se as the analogous N2S species22 was observed at 2040 cm-1, but no new absorption was observed. Apparently, N2Se is not stable enough to be formed in these experiments. Acknowledgment. We gratefully acknowledge financial support from NSF Grant CHE 91-22556 and computer time at the San Diego Supercomputer Center. References and Notes (1) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry; WileyInterscience: New York, 1988. (2) Milkulski, C. M.; Russo, P. J.; Saran, M. S.; MacDiarmid, A. G.; Garito, A. F.; Heeger, A. J. J. Am. Chem. Soc. 1975, 97, 6358. (3) Teichman, R. A., III; Nixon, E. R. Inorg. Chem. 1976, 15, 1993. (4) Findlay, R. H.; Palmer, M. H.; Downs, A. J.; Edgell, R. G.; Evans, R. Inorg. Chem. 1980, 19, 1307. (5) Lau, W. M.; Westwood, N. P. C.; Palmer, M. H. J. Am. Chem. Soc. 1986, 108, 3229. (6) Chivers, T. Acc. Chem. Res. 1984, 17, 166. (7) Oakley, R. T. Prog. Inorg. Chem. 1988, 36, 299. (8) Parsons, S.; Passmore, J. Acc. Chem. Res. 1994, (9) Jander, J.; Doetsch, V. Chem. Ber. 1960, 93, 561. (10) Baker, C. K.; Cordes, A. W.; Margrave, J. L. J. Phys. Chem. 1965, 69, 334. (11) Gowik, P. K.; Klapotke, T. M. Spectrochim. Acta 1990, 46A, 1371. (12) Awere, E. G.; Passmore, J.; White, P. S. J. Chem. Soc., Dalton Trans. 1993, 229. (13) Carrington, A.; Howard, B. J.; Levy, D. H.; Robertson, J. C. Mol. Phys. 1968, 15, 187. (14) Amano, T.; Saito, S.; Hirato, E.; Morino, Y. J. Mol. Spectrosc. 1969, 32, 97. (15) Matsumura, K.; Kawaguchi, K.; Nagai, K.; Yamada, C.; Hirota, E. J. Mol. Spectrosc. 1980, 84, 68. (16) Subbaram, K. V.; Rao, D. R. Chem. Phys. Lett. 1970, 4, 653. (17) Jones, W. E.; Harding, L.; Tee, K. K.; Jenouvrier, A.; Daumont, D.; Pascat, B.; Guenebaut, H. Can. J. Phys. 1971, 49, 2033. (18) Jenouvrier, A.; Pascat, B.; Lefebvre-Brion, H. J. Mol. Spectrosc. 1973, 45, 46. (19) Brown, J. M.; Uehara, H. J. Chem. Phys. 1987, 87, 880. (20) Wentrup, C.; Fischer, S.; Maquestiau, A.; Flammang, R. J. Org. Chem. 1986, 51, 1908. (21) Bender, H.; Carnovale, F.; Peel, J. B.; Wentrup, C. J. Am. Chem. Soc. 1988, 110, 3458. (22) Hassanzadeh, P.; Andrews, L. J. Am. Chem. Soc. 1992, 114, 83. (23) Yamaguchi, Y.; Xie, Y.; Grev, R. S.; Schaefer, H. F., III J. Chem. Phys. 1990, 92, 3683. (24) Kaldor, U. Chem. Phys. Lett. 1991, 185, 131.
J. Phys. Chem., Vol. 100, No. 41, 1996 16673 (25) Andrews, L.; Hassanzadeh, P. J. Chem. Soc., Chem. Commun. 1994, 1523. (26) Brabson, G. D.; Andrews, L. J. Phys. Chem. 1992, 96, 9172. (27) Schnockel, H.; Koppe, R. J. Am. Chem. Soc. 1989, 111, 4583. (28) Mielke, Z.; Brabson, D.; Andrews, L. J. Phys. Chem. 1991, 95, 75. (29) Brabson, G. D.; Andrews, L.; Marsden, C. J. J. Phys. Chem. 1996, 100, 16487. (30) Hauschild, E. A.; Kannewurf, C. R. J. Phys. Chem. Solids 1969, 30, 353. (31) Lakshminarayana, G.; Shetty, B. J. J. Mol. Spectrosc. 1988, 130, 155. (32) Brabson, G. D.; Mielke, Z.; Andrews, L. J. Phys. Chem. 1991, 95, 79. (33) Anderson, J. S.; Ogden, J. S. J. Chem. Phys. 1969, 51, 4189. (34) King, G. W.; Srikameswaran, K. J. Mol. Spectrosc. 1969, 29, 491. (35) Gaussian 94, Revision B.1, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian, Inc., Pittsburgh, PA, 1995. (36) Stevens, P. J.; Devlin, F. J.; Chablowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (37) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639. Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033. Hay, P. J. J. Chem. Phys. 1977, 66, 4377. Raghavachari, K.; Trucks, G. W. J. Chem. Phys. 1989, 91, 1062. (38) Schlegel, H. B. J. Comput. Chem. 1982, 3, 214. (39) Allavena, M.; Rysnik, R.; White, D.; Calder, V.; Mann, D. E. J. Chem. Phys. 1969, 50, 3399. (40) Banister, A. J.; Hey, R. G.; MacLean, G. K.; Passmore, J. J. Inorg. Chem. 1982, 21, 1679. (41) Andrews, L. Annu. ReV. Phys. Chem. 1979, 30, 79. (42) Warren, D. S.; Zhao, M.; Gimarc, B. M. J. Am. Chem. Soc. 1995, 117, 10345. Assignment of the 1167.5 cm-1 absorption (ref 22) to the pairwise ring S2N2 species is not appropriate because of the lack of a triplet in the mixed 14/15 nitrogen isotopic experiment. (43) Kelly, P. F.; Slawin, A. M. Z. Angew. Chem., Int. Ed. Engl. 1995, 34, 1758. (44) Hacaloglu, J.; Suzer, S.; Andrews, L. J. Phys. Chem. 1990, 94, 1759. (45) Hawkins, M.; Downs, A. J. J. Phys. Chem. 1984, 88, 3042. (46) Andrews, L.; Hassanzadeh, P.; Brabson, G. D.; Citra, A.; Neurock, M. J. Phys. Chem. 1996, 100, 8273. (47) Varetti, E. L.; Pimentel, G. C. J. Chem. Phys. 1971, 55, 3813. (48) Schnockel, H. Angew. Chem., Int. Ed. Engl. 1980, 19, 323.
JP961288A
