1646
David E. Tevault and Lester Andrews
Matrix Reactions of Sodium, Potassium, Rubidium, and Cesium Atoms with Nitric
Oxide. Infrared Spectra of the M+(NO)- Species David E. Tevault and Lester Andrews* Chemistry Department, University of Virginia. Charlottesviile, Virginia 22903 (Received February 9, 1973)
The 15°K condensation of beams of Na, K, Rb, and Cs atoms with NO a t high dilution in argon produced two intense product bands for each metal reaction product. Absorptions in the 1358-1374-cm-l region were assigned to the intraionic (N-0)- mode and bands in the 361-219-cm-l region were assigned to interionic M+-(NO)- vibration. The small M + effect on the (N-0)- mode is explained by the ionic model of polarizable ion pairs. The large 15N shift for vz of the heavy metal species suggests a bent or linear M+NO- arrangement in contrast to the acute triangular Li+(ON)- structure.
Introduction The previous paper1 describes a reinvestigation of the lithium-nitric oxide matrix reaction and of the structure, photochemistry, and bonding in the principal product species LiON and Li(0N)Li. The position of the N-0 frequency of the Li+(ON)- species in the NO- spectral region invites consideration of the other M f ( 0 N ) - species and the resulting M + effect on the NO- frequency. An interesting trend has been observed for 0 2 - frequencies in the M + O y species;2 the 0 2 - fundamental increases over a 20-cm-' range with increasing M + size.3 This trend has been explained in terms of an ionic model3 of polarizable ion pairs, M+Oz-. The ionic model for sodium nitroxide has received support from recent a b initio molecular orbha1 calculations.* In order to test the ionic model hypothesis, it was desired to obtain the NOfrequencies for all the M + (ON) - species. Milligan and Jacox5 recently reported Na and Cs atom matrix reactions with NO; these workers assigned frequencies of 1359 and 1374 cm-I to the Na-k(ON)- and Cs+(ON)- species. However, the region between 200 and 400 cm-1, which also contains important spectral features due to these species, was not examined. We report here a complete study of matrix reactions of Na, K, Rb, and CS atoms with isotopic nitric oxide species with infrared spectroscopic examination in the 200-2000-~m-~spectral region before and after mercury arc photolysis. '
Experimental Section The instrumentation and experimental techniques have been described in previous publications from this laboratory.l.6 Nitric oxide and isotopically enriched nitric oxides were handled as described in the previous paper;l matrix samples of NO and 15NO a t Ar/NO = 500 were used whereas mixed N16J80 samples were made to a total Ar/NO = 300/1 ratio. Elemental sodium and potassium were cut and loaded into Knudsen cells in an argon atmosphere. Rubidium and cesium atom beams were produced from the reaction of RbCl and CsCl with melted lithium in the Knudsen cell. Typical Knudsen cell operating temperatures were 225" for Na, 165" for K, 300" for RbC1-Li, and 320" for CsC1-Li. These temperatures produce approximately 1-p vapor pressures7 for Na and K and for Rb and Cs as well. The Journal of Physical Chemistry, Voi. 77, No. 73, 1973
Results In all of these experiments, NO and (NO)z bands were observed in the 1700-1900-cm-1 region along with traces of water and carbon dioxide, When sodium atoms were codeposited with nitric oxide-argon samples, Ar/XO = 500/1, two prominent new features were observed, as Figure 1 and Table I indicate. The sharp intense band a t 1358.0 cm-1 is in good agreement with the 1358.6-cxr1 band of Milligan and Jacox; the broad absorption a t 361 cm-I was not reported by these workers.5 In isotopic substitution experiments the sharp band shifted to 1335.7 cm-I for 15N0 and 1323 cm-1 for NlsO; these features agree with the data of Milligan and Jacox.5 The lower frequency band shifted to 357 cm-I with l5NO and 358 cm-I with N160/N180 = 2/1 precursors, although accurate frequency measurement of this broader feature is more difficult. The reaction of potassium atoms with nitric oxide likewise yielded only two bands; intense features a t 1372.0 and 280.4 cm-1 are shown in Figure 1. Andrews6 has demonstrated that simultaneous deposition of two different alkali metals provides a reasonable mixed "pseudo" isotopic experiment for the heavy alkali atoms. Accordingly the mixed Na-K reaction with NO depicted a t the top of Figure 2 was studied. Note that four bands appeared: the two observed in separate Na reactions and the two produced in K experiments. The observation of two doublets in this mixed metal experiment indicates that these absorbers contain single alkali metal atoms. Similar conclusions were reached from. mixed Li-Cs and 6Li-7Li experi; ments in the previous paper.l Figure 2 also contrasts the reactions of K atoms with natural isotopic NO, 15NO, and N160/N180 = 1/1 samples. The sharp intense feature was shifted 23.6 cm-l to 1348.4 cm-I with 15NO and 35.2 cm-I to 1336.8 cm-l with N1SO. The lower frequency band shifted 2.6 cm-1 to 277.8 cm-I upon 15N substitution and 1.6 cm-l to 278.8 cm-1 using equimolar N160-Nl80. Hence, a 3-cm-' ISO (1) D. E. Tevault and L. Andrews, J. Phys. Chem., 77, 1640 (1973). (2) L. Andrews, J. Chem. Phys.. 50, 4288 (1969). (3) L. Andrews and R. R. Smardzewski, J. Chem. Phys., 58, 2258 (1973). (4) J. Peslak, J r . , D, S. Klett, and C . W. David, J. Chem. Phys., 5 5 , 1993 (1971). (5) D. E. Miiligan and M . E. Jacox, J. Chem. Phys.. 55, 3404 (1971). (6) L. Andrews, J. Chem. Phys., 54, 4935 (1971). (7) W. T. Hicks, J. Chem. Phys., 38, 1873 (1963).
1647
Infrared Spectra of the M+(NO) - Species
Z
0
z
2
cn
cn
0 E
2
a
siz
I-
[11
v)
Z
a
(L:
I-
+ z
IZ W
W
V
(L:
V
a
W
[11
W
a 1 I300
Figure 1. Infrared spectra in the 200-400- and 1320-1400cm-I regions for matrix reactions of Na, K, Rb, and Cs atoms with nitric oxide diluted in argon, Ar/NO = 500. Sample deposition temperature 15°K. TABLE I: Absorptions (cm-l) Observed Following Matrix
Reactions of Heavy Alkali Metal Atoms with Isotopic Nitric Oxide Diluted in Argon Isotopic sample
NO 1 5 ~ 0
"Boa
I
1
I
"
I A I
1320
I
K
Rb
CS
1358.0 361 1335.7 357 1323.8 358
1372.0 280.4 1348.4 277.8 1336.8 278.8
1373.0 235.0 1349.6 231.1 1338.0 234.5
1374.0 219.0 1350.1 214.8 1338.8 218.0
shift is deduced for the lower frequency K feature from the N160-N1S0 isotopic mixture. The mixed oxygen isotopic spectrum in Figure 2 shows a well-resolved doublet in the 1 3 0 0 - ~ m -region ~ indicating a single oxygen atom motion and implying the incorporation of a single nitric oxide molecule in this new molecular species. The lower frequency band could not be resolved owing to the large bandwidth relative to the small isotopic shift. Isotopic frequency data are listed in Table I. Figure 1 illustrates the matrix reaction of rubidium atoms with nitric oxide using Ar/NO = 500. Again, only two new features were observed, a sharp intense band at 1373.0 cm-I and an intense feature at 235.0 cm-l. In an experiment using ISNO, these features shifted to 1349.6 and 231.1 cm-1; in an equimolar 14NO-15N0 experiment the 1373- and 1350-cm-l features formed a well-resolved doublet, but the lower broad 233-cm-l band was not resolved. The doublet indicates the presence of a single nitrogen atom in the species responsible for the higher frequency band, which implies the presence of one NO molecule in the new product species, the same conclusion reached from niixed oxygen isotopic experiments. In an experiment using mixed N160/N180 = 3/1, the upper bands were observed a t 1373.0 and 1338.0 cm-1 and the lower band at 234.5 cm-1. The cesium atom reaction with NO produced only two bands, an intense feature at 1374.0 cm-I in agreement with the Milligan-Jacox observation5 and an intense band a t 219.0 cm-1. The isotopic experiments on the cesium species produced bands at 1350.1 and 214.8 cm-I using
I
1
240
FREQUENCY (cm-' )
Figure 2. Infrared spectra in the 220-380- and 1300-1400cm spectral regions for matrix reactions of alkali metal atoms with nitric oxide at high dilution in argon: top spectrum, equimolar Na/K, Ar/NO = 400; second spectrum, K , with Ar/NO = 500; third spectrum, K with Ar = 500; bottom spectrum, K with A r / N 1 6 0 / N 1 8 0 = 600/l/:.5N0
Metal 6Li
7Li
Na K Rb
aSamples mixed N160/N180; ratios were 2/1 for Na, 1 / 1 for K , 3/1 for Rb, and 2/3 for Cs.
I
300
TABLE II: Frequencies (cm-l) Assigned to up Interionic Modes of M + ( O N ) - Speciesa
Na
I
I
1
360
cs
VI
1353.5 1352.5 1358.0 1372.0 1373.0 1374.0
u1
lntraionic and Y2 of M+02-
v2
692.3 651 361 280.4 235.0 219.0
.a
'
743.8 698.8 390.7 307.5 255.0 236.5
a v 2 Of the Mt02- species are listed for comparison. L. Andrews, eta/., J. Phys. Chem., 77, 1065 (1973), see also ref 2 and 6.
15NO and 1338.8 and 218.0 cm-1 using 60% oxygen-18 enriched NO. These frequency data are listed in Table 11. Most of the above heavy alkali metal-nitric oxide matrix samples were subjected to high-pressure mercury arc photolysis. In contrast to the previous1$5 photolysis behavior of Li+(ON)-, the two absorptions discussed above for each metal reaction sustained no discernible intensity or frequency changes upon photolysis. Milligan and Jacox reported5 that the higher frequency bands in Na and Cs experiments showed no changes with photolysis.
Discussion In the previous paper, evidence was presented associating two absorptions with intraionic and interionic vibrational modes of the Li+(ON)- molecular species. The frequencies presented above are appropriate for the analogous heavy alkali metal species. The mixed Na-K experiment illustrated at the top of Figure 2 shows doublets a t 1358-1372 and 361-280 cm-I without intermediate components; this observation indicates that the molecular species absorbing in these two spectral regions contains a single alkali metal atom. Mixed 14J5NO and N16J80 experiments indicate that the upper absorption arises from a single NO species; the lower frequency bands could not be resolved owing to the much smaller isotopic shifts. The above data indicate a molecular formula MON for the new species. The Journalof Physical Chemistry, Vol. 77,No. 13, 7973
David E. Tevault and Lester Andrews
1648 TABLE Ill: Nitrogen and Oxygen Isotopic Shifts ( c m - ' ) for the v 2 Interionic Modes of the M+(ON) - Speciesa Alkali metai
%i 7Li
Na K Rb
cs
TABLE IV: Ionic Model Dipole Moment Calculations for the M + N O - Molecules
Au2 '80for A u 15N ~
AZJP "0
M+'60'80
0.8 0.8 (4) 2.6 3.9 4.2
6.3 5.8 (6) 3c 2c 2C
3.0 3.1 4.0
Li+NO-
NafNOK+NO Rb+NO-
5 6.5 6.8
Cs+NO-
Single oxygen-I8 isotopic shifts of M'O2- are listed for comparison. Data less accurateiy known due to difficulty in accurately measuring broader bands. Deduced from mixed N ' 6 0 - N ' 8 0 experiments. a
The bands in the 1300-wave number region show isotopic shifts appropriate for a fundamental N-0 vibration. For the potassium case illustrated in Figure 2, the observed 15N shift was 23.6 cm-1 while the diatomic harmonic oscillator calculation predicts 24.5 cm-1; the observed I80 shift was 35.2 cm-I and the diatomic shift is 36.1 cm-I. These small 0.9-cm-l discrepancies could be due t o anharmonicity or to a very small amount of alkali metal participation in the vibrational mode. As pointed out by Milligan and Jacox,5 the proximity of these bands to the 1355-cm-l NO- fundamental deduced from lowenergy electron impact studies8 indicates that these molecular species contain the NO- moiety. The present data show that this NO- anion is coulombically bound to an alkali cation forming a M+(ON)- species. The small alkali metal shift confirms this point. The upper frequency bands are assigned to v1 the intraionic (N-0)- mode in the M+(ON)- species. The lower frequency band is clearly appropriate for an (ON)-. Note the large alkali interionic mode M + metal shift which closely follows the analogous M+ 0 2 - mode, as is compared in Table I1 for v2 frequencies, and the small 15N and 1 8 0 shifts which are listed in Table I11 for the vz M +(Or\;)- modes. Any force constant calculations for the M+NO- molecules must involve approximations since only two of the three normal modes were observed. Although the M+NO- diatomic approximation using NO as a single mass worked well for Li+ON-,1 this approach is not successful for the heavier M"N0- species. This is expected from comparison of the data of Table 111. In the Cs+NO- case, IQ is predicted at 216 cm-1 for 133-15-16 isotopes and 213 cm-1 for 133-14-18. This represents too much 0-18 shift and insufficient Tu'-15 shift. If v 2 were a Cs-0 mode, the simple diatomic approximation predicts an 11-cm-1 0-18 shift; 2 cm-1 was observed. Likewise, if v 2 were a simple Cs-N mode, a 8-cm-l shift is predicted whereas 4 cm-1 was observed. Clearly ~2 is a mode involving both N and 0, but the increased N-15 shift and decreased 0-18 shift indicates a greater participation of N in v 2 of Cs+NOrelative to v2 of Li+ ON- , Interpretation of the above infrared observations as arising from an M+(ON)- species raises two interesting points: the 22-cm-1 dependence of the NO- frequency upon M+ and the structure of the M+(ON)- species. Now, the alkali metal influence on the NO- frequency must be put into proper perspective. Upon going from NO to the NO- anion, an antibonding electron is added which reduces the fundamental frequency more than 500 cm-I;
-
The Journal of Physicai Chemistry. Vol. 77, No. 73, 1973
-
1.83 2.19 2.47 2.58 2.68
8.79 10.51 11.86 12.39 12.88
3.50 2.19 2.56 2.77 3.24
5.29 8.32 9.32 9.62 9.64
0.601 0.791 0.786 0.777 0.749
a Interionic distance r estimated to be average of M - F - and MtCIgas-phase internuclear distances plus 2%. NO- anion polarizability estimated using NO polarizability data, the assumption that & ( N O - ) / a ( O n - ) = a ( N O ) / a ( 0 2 )and the solution for 011 described in ref 3. A ~ NO-. value of 2.41 A3 was used for a . of
this contrasts the 22-cm-1 spread from Li+ up to Cs+. Clearly, the increase in (NO)- frequency with increasing M+ mass cannot be a mass effect since such an effect would be expected to lower the (NO)- frequency. The Rittner ionic model of polarizable ion pairs9 has been recently applied to the M+Oz- specie^.^ The first two terms of Rittner's equation for the dipole moment g , a measure of charge separation and hence antibonding electron density on NO-, are as follows p = er - -ace - - a,e (1) r2 r2 ion dipole induced dipoles This treatment can only be interpreted qualitatively because r, the interionic distance, and cia, the NO- anion polarizability, are not accurately known. Using reasonable values of r and a,, dipole moments for the M+NOspecies were calculated; values are listed in Table IV. The last column in the table represents the fractional ionic character which is a measure of the charge transferred from M to NO. With the exception of the Li+ species, which is the difficult one to calculate for Li+F- and LifC1- as well,3 the fractional ionic character decreases from the Na+NOspecies to the Cs+NO- species. The important point here is the qualitative trend not the quantitative values; as the alkali cation polarizability, cyc increases, the induced dipole on Mf increases which opposes the ion-dipole charge transfer and decreases the fractional ionic character and the antibonding electron density on NO-. Accordingly the NO - frequency increases with increasing alkali cation polarizability. Similar behavior has been observed for 0 2 as a function of M + in the M+Oz- species.3 Clearly, the ionic model of polarizable ion pairs provides an adequate explanation for the small frequency increase with increasing cation size. Three observations invite consideration of the M+(ON)- structure. First, the v p isotopic shifts change from a major 1 8 0 dependence and minor 15K shift with Li+ to more 15N shift than 1 8 0 effect in the Cs+ species. Secondly, no heavier M(0 N ) M secondary reaction products were observed, in contrast to the heavier MOzM peroxides. Finally, photolysis did not change the band intensities or, accordingly, the M+(ON)- structure which contrasts the Li+(ON)- photolysis behavior.1,5 Although a definite conclusion cannot be reached, the above observations suggest a different structural arrange(8) D. Spence and G .J. Schulz, Phys. Rev. A, 3, 1968 (1971) (9) E. S.Rittner, J. Chem. Phys., 19, 1030 (1951).
Infrared Spectra of the Ammonia-Hydrochloric Acid Cdmplex ment for the K + , Rb+, and &+(NO)- species as compared to the acute triangular models for Li+(ON)- and M+Oz-. The breadth of the 361-cm-1 Na+(NO)- band may indicate that more than one cation-anion arrangement is matrix isolated for this species; the isotopic shift data for the 361-cm-1 band (see Table 111) are not conclusive on suggesting whether Na+NO- is most likely like Li+ON- or K+NO-. The large 15N shift and small shift data for the K + , Rb+, and Cs+ species suggest a bent or linear M+(NO)- arrangement for the three heaviest alkali metals, in contrast to the triangular models proposed134 for Li+(ON)- and Na+(ON)-. It should be noted that the K + ion is comparable in size to NO-, and Rb+ and Cs+ are larger than NO-; hence, a different structural arrangement for these larger cations may not be too unreasonable. If the K+(NO)-, Rb+(NO)-, and Cs+(NO)- species are produced in a stable bent or linear form, photolysis cannot effect isomerism, which rationalizes the photolytic stability of the observed bands. Perhaps the open M + ( N 0 ) - structure is less susceptible to secon-
1649
dary reaction forming a two metal species, than the triangular Li+(ON)- or M+Oz- species.
Conclusions When Na, K, Rb, and Cs are deposited with matrix samples of NO, two intense product bands appear which correspond to the two most intense features in lithiumnitric oxide reactions. The higher frequency in the 13581374-cm-1 range is assigned to V I , the intraionic (N-0)mode and the lower frequency in the 361-219-cm-l region is assigned to V Z , the interionic M+-(NO)- motion. The M* effect on the NO- mode is explained by the Rittner ionic model of polarizable ion pairs. The large I5N shifts for u2 of the heavy metal species suggest the bent or linear structures M+NO- for the K + , Rb+, and Cs+ species, in contrast to the triangular Li+(ON)- species. Acknowledgment. The authors gratefully acknowledge financial support of this research by the National Science Foundation under Grant No. GP-28582.
Infrared Spectra of the Ammonia-Hydrochloric Acid Complex in Solid Nitrogen Ejruce S. Ault
and George C.
Pimentel"
Department of Chemistry, University of California, Berkeley, California 94720 (Received February 20, 7973) Publication costs assisted by the U.S. Office of Naval Research
The infrared spectrum was obtained for the 1:l complex of NH3 and HCl isolated in a nitrogen matrix at 15°K. The NH3.HCl complex absorbs a t 630, 705, 1246, and 1438 cm-l, while bands of the perdeuterio counterpart ND3-DC1 were identified a t 470 and 505 cm-1. The spectra of the mixed isotopic species indicate that the hydrogen atoms of the complex are not equivalent. Comparison to known spectra of prototype compounds shows that a very strong hydrogen bond is formed, involving specifically the hydrogen atom attached to the HC1 molecule. However, this complex contains a hydrogen bond in which the hydrogen atom of the HCl is nearly equally shared by the chlorine and the nitrogen. These results are compared to Clementi's extensive theoretical calculations on this complex, with very good agreement. Thermodynamic arguments about the stability of an isolated NH3.HC1 complex support these conclusions as well.
Introduction Recently Ault and Pimentell reported the infrared spectrum of the hydrogen bonded complex formed between HzO and HCl in a nitrogen matrix a t 20°K (hereafter, this paper will be called AP). The spectra show that HC1 forms a hydrogen bond in which its hydrogen remains chemically distinct from the hydrogens of the base, HzO, and that the ion pair, H30+.C1-, does not form. We report here a parallel study of the ammonia-hydrochloric acid complex in solid nitrogen. This system was selected because the higher base strength of NH3 should encourage more complete transfer of the hydrogen-bonded proton and, thus, reveal the extent to which ion pair formation, NH4+ .C1-, depends upon stabilization by an ionic lattice.
The H3N. HC1 molecular complex has been detected by Goldfinger and Verhaegenz using high-temperature mass spectrometry. The results give no clue to the structure of the complex. Ciementi has made an intensive set of ab initio LCAO-SCF calculations on H3N HC1.3-5 These calculations, which omit correlation energy, provide an estimate of the bond energy (19 kcal/mol) and they indicate a partial proton transfer. Without experimental validation, the significance of these calculations is difficult to assess. The spectra we have obtained provide the needed experimental tie point. (1) (2) (3) (4) (5)
B. S. Ault and G . C. Pimentel, J. Phys. Chem., 77, 57 (1973). P. Goldfinger and G . Verhaegen, J. Chem. Phys., 50, 1467 (1969). E. Ciementi, J. Chem. Phys., 46, 3851 (1967). E. Ciementi, J. Chem. Phys., 47, 2323 (1967). E. Clementi and J. N. Gayies, J. Chem. Phys., 47, 3837 (1967).
The Journal of Physical Chemistry, Voi. 77, No. 13, 1973
