COMMUNICATIONS TO THE EDITOR
454
Acknowledgment. This work was supported by the National Institute of General Medical Sciences under Grant No. GM-12596. The authors also gratefully acknolvledge the support of R. W. T. by a National Science Foundation Graduate Traineeship. DEPARTMENT OF CHEMISTRY WAYNE STATEUNIVERSITY DETROIT,MICHIGAN48202
fi. W. TAYLOR D. B. KORABACHER*
RECEIVED OCTOBEI~ 1, 1971
Anomalous Isotope Shifts in the Vibrational Spectrum of Hydrogen Cyanide in Argon Matrices' Publication costs ossisted by Amea Lnboratow, U. S. Atomic Energy Commission
Figure 1. Ileiszwtion CIIYVVCSfor the a q ~ i w u s cobaIt(II)-arnmonia system obtained a t pII (a) 3.36 (top), (b) 5.65 (center), (e) 5.96 (bottom). All solutions contain 0.020 M Co(ClO&, 0.10 M NHd2104, 8 X 10-6 M bromocresol green plus NaClO, to achieve 0.2 M ionic strength. Conditions for each spectrum are: AT = 4.6' (find temp = 25'), sweep rate = 0.5 msec/div, amplitude = 5 mV/div, h = 592 nm.
concentration before and after the temperature-jump, respectively. I n view of the widespread use of coupled indicator systems in relaxation studies, the bicipital relaxation phenomena described above suggest that systems which may previously have been abandoned as unobservable should be reexamined under varying solution conditions to ascertain whether the difficulty may be attributable to the inadvertent use of conditions close to the null relaxation point. I n addition, investigators working with coupled systems might be well advised to establish the existence of a null relaxation point as confirming evidence that only a single relaxation is occurring in t,he observed time range, it beink extremely unlikely that identical null relaxation points would he exhibited by closely overlapping relaxation processes which might otherwise go undetected. The J o u m l of Physical Chemistry, Vol. 76, No. 5,197s
Sir: The matrix spectra of isotopic species of small molecules have been used widely to deduce the molecular geometry of labile molecules via the Redlich-Teller product rule.* Structures so obtained correspond to the geometry of the free molecule only if the vibrational model of t,he free molecule adequately describes the molecular vibrations in the matrix. If this prerequisit,e is fulfilled,the isotopic shift,sof t,hn vibrational frequencies in the matrix spectra will be the same as those in the gas phase vibrational spectra; if it is not fulfilled, the isotope shifts will differ. Verification of this prerequisite is crucial to the validity of mat,rix isolation structural studies because small changes in the isotopic frequency shifts in the matrix would be incorrectly interpreted in terms of the geometry of the molecule rather than a& tributed to the use of an inappropriate vibrational model. The gas phase and matrix spectra of only a few molecules have been nufficiently well characterized to provide an unambiguous test. of this prerequisite. The isotopic species of HCN offer one such case. Careful measurement of the infrared spectra of HCN isotopes isolated in an argon matrix at 8°K were made using a Perkin-Elmer E-13 monochromator. The concentration of the samples varied from 0.0015 to 0.0030 mole ratio HCN:Ar. All the fundamentals were ohserved except the v1 vibration of the hydrogen isotopes. The intensity of this vibration is very weak; however upon deuteration the intensity increases markedly so that the vibration was observed in the deuterium isotopes. In addition certain other features attributable to dimers and polymers3 were observed. The assignment of the monomer absorptions was made by analogy (1) Work w a performed in the Ames Laboratory of the Atomic Energy Commission. Contribution No. 3110. (2) G . Herzberg, "Moleoular Spectra and Molecular Structure. 11. Infrared and Hsman Spectra of Polyatomic Molecules," Van Nostrand, Princeton, N. J., 1945. p 227 ff. (3) Charles M. King and Eugene K. Niaon, I.Chem. Phya., 48, 1685 (1963).
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455
Table I : Vibrational Frequencies of HCN Isotopes in Argon Matrices and the Cas Phase Y1
H12C14N H 13C14N H12C15N D12C14N D 13C* 4N D12C15N a
-
Matrix
7
1925.17 1911.91 1900.16
A.D.O
0.03 0.01
0.01
Average deviation from the mean.
4 3 3 b
GasC
7
Ye
AB."
No.b
US
A.D.O
NO.^
720.96 714.94 719.74 576.02 568.01 574.44
0.05 0.03 0.04 0.05 0.06 0.02
10 8 4 4 4 6
3305.66 3288.08 3304.61 2626.43 2585.85 2616.99
0.07 0.13 0.10 0.05 0.03 0.02
4 6 4 4 6 4
No!
Number of times the frequency was calibrated.
vi
2096.85 2063.05 2064.35 1925.24 1911.81 1900.12
ut
YZ
712.35 706.34 711.41 569.30 561.60 568.06
3311.45 3293.46 3310.13 2630.34 2590.05 2621.22
From ref 3 and citations therein.
Table I1 : Isotope Shifts of HCN Vibrational Frequencies in the Matrix and Gas and the Discrepancy between Them I-
Matrix
-Av,----------
Gas
7
6
Matrix
hv-
Gas
Avi
7
8
7
Matrix
Gas
17.53
17.99
-0.46
1.00
1.32
-0.32
5
H12Cl4N Hl3CI4N H1aC14N
6.02
6.00
1.22
0.94
0.02 -0.28
H zC16N D12C14N 13.26
13.43
-0.17
8.01
7.70
0.31
40.58
40.29
0.29
25.01
25.12
-0.11
1.58
1.24
0.24
9.44
9.12
0.32
144.94
143.05
1.89
679.18
681.11
-1.93
146.93
144.74
2.19
702.23
703.41
-1.18
145.30
143.35
1.95
687.62
688.91
-1.29
DW14N Dl2Cl4N Dl2Cl6N H12C14N D12C14N HlaC14N DlaC14N H12C16N D12C16N
to the location of the bands in the gas phase, by diffuWhile these inconsistencies have little effect on the sion experiments, and by observing the relative intenvalue of the force constants for HCN evaluated from sities of features as a function of concentration. Each the matrix spectra, the effect on the calculated molecfeature was calibrated from three to ten times using ular geometry using the product rule is much more seHCN, DCN, and in one case, HzO vapor as a ~ a l i b r a n t . ~ rious. Because of the sensitive dependence of the calThe spectral resolution was 1.5,0.7, and 0.3 cm-' in the culation on the isotopic shift, an error of even a few 3000, 2500, and 700 cm-' regions, respectively. The tenths of a cm-I represents a substantial uncertainty in experimental data, the number of calibrations, the the calculated geometry. For example, application of average deviation of the calibrations from the mean, the Redlich-Teller Product Rule to the bending vibraand the gas phase band centerss are summarized in tion of HCN in the matrix yields ratios of the CN and Table I. CH bond lengths that are not only mutually inconsisThe isotopic shifts of the matrix and gas phase fretent but also are in serious disagreement with the analquencies and the discrepancies between them, 6, are ogous computation using the gas phase data. As tabulated in Table I1 for various isotopic combinations. The discrepancies are small for isotopic combinations (4) K. N. Rao, C . J. Humphreys, and D. H. Rank, "Wavelength involving mass changes of the heavy atoms. On the Standards in the Infrared," Academic Press, Washington, D. C., other hand, for isotopic combinations involving H and 1966; P. Fraley and K. N. Rao, J. Mol. Spectrosc., 26, 312 (1969); P. B. Checkland and H. W. Thompson, Trans. Faraday Soc., 51, 1 D, the discrepancies are about 2 cm-'. This far ex(1955). ceeds the combined uncertainty of the gas and matrix (5) T . Nakagawa and Y. Morino, Bull. Chem. Soc. Jap., 42, 2212 data. (1969). The Journal of Physical ChemGtry, Vol. 76, No. 3, 1978
COMMUNICATIONS TO THE EDITOR
456 shown in Table 111,no improvement in the value of the bond length ratio in the matrix accrues when anharmohicity is compensated for by applying the appropriate anharmonic constants obtained from the overtone-combination analysis in the gas phase16although the gas phase data shomd excellent consistency whcn this correction was applied.
Table I11 : Ratios of Gas Phase and Matrix Bond Lengths Calculated from the Observed and Harmonic Frequencies Using the Redlich-Teller Product Rule ---Gasa--7 -
Matrix---
--(TCN/TCH)---
Observed
13armonio
Observed
Harmonk
1.01
0.96 i 0.03
1.145
1.079
0.99
0.97 f 0 . 0 3
1.116
1.082
1.13
1.09 =k 0.02
1.119
1.081
1.07
1.05 f 0.02
1.095
1.081
0.91
0.85 3~ 0.05
1.169
1.077
0.93
0.89 i 0.05
1.135
1.082
1.00
0.97 =k 0.04
1.130
1.080
--(TcN/TcH)----
Microwave measurements give a value 1.083 for this ratio.
It does not seem plausible that the discrepancies of the isotope shifts can be accounted for by a change in the geometry of HCK in argon. The van der Waals forces that exist between the molecule and the matrix environmect are not strong enough to deform the HCN bonds 0.1 A. The discrepancy most likely results from the failure of the linear x-y-x vibrational model to describe the vibrations of the matrix isolated molecule adequately. If this is true, one is forced to conclude that molecular geometries based on the analysis of isotopic shifts in matrices are likely to be in serious error when light isotopic substitutions are involved, e.g., H and D, no matter how precisely the frequencies are measured and no matter how well anharmonicity is accounted for. INSTITUTE FOR ATOMIC RESEARCH DEPAR.TMENT OF CHEMISTRY IOWA STATEUNIVERSITY AMES,IOWA50010 AND
Polywater Publication costa assisted by the Naval Research Laboratory
Sir: Recently, Brummer, et aL,l gave a detailed description of their procedure for preparing “polywater” or anomalous water in high yields. Their results may be interpreted as merely adding to t,he evidence that this material is not a modified or polymeric form of water but is simply the corrosion product of the Pyrex or quartz capillaries in which it is formed as suggested by Bascom, Brooks, and Worthington.2 One of the principal points made by Brummer, et al., is that the yield of “polywater” is greatest in strained regions of the glass or quartz capillary. For example, their yield was particularly high at the tapered ends of thick walled capillaries that had been sealed by melting and drawing. Annealing the capillaries removed this enhanced activity. In the earlier work of Bascom, et U Z . , ~ it was pointed out that strained siloxane bonds would be especially susceptible to attack by adsorbed mater made alkaline by cations present in the Pyrex (or in the case of quartz by electrolytes that had surface-diffused into the capillaries). The product of this surface corrosion would be an alkaline silicate sol or gel. Brummer, et al., and others before them3 claim that the infrared spectrum of “polywater” is unique and cannot be attributed to inorganic salts. However, Bascom, et al., have shown that spectra of bicarbonatesilicate mixtures closely resemble the polywater spectra. They argue that the alkaline condensate formed in glass and silica capillaries absorbs atmospheric COZ to form the bicarbonate ion. An example of the bicarbonatesilicate spectra is given in Figure 1. It closely resembles the spectrum of the Pyrex-grown polywater given by Brummer, et al. (Figure 2C, reference 1). The bands at! 1650 cm-l and 1425 cm-’ originally attributed to “ p ~ l y w a t e r ”are ~ due to the symmetric and antieymmetric 04-0 stretching vibrations. The band at 1000-1100 cm-l is due to the Si-0-Si stretch of the silicate constituent. This band and the sharp bands between 800 and 900 cm-l are also present in the “polywater” It should also be noted that the 1425-cm-’ band appears to be a doublet in both the bicarbonate-silicate spectrum and the “polywater” spectra. The salt solution used to obtain Figure 1 was made by mixing equal amounts of a 10% (by weight) solution of KHC03 and a 15% solution of SiOz and Na~Si03at a 3.3: 1 mole ratio. This mixture was diluted to about 0.5% with distilled water. Approximately 0.25 ml of this final solution was then dried to a gel on an Irtran
JACOB PACANSKY (1) 8 . B. Brummer, G. Entine, J. I. Bradspies, H. Lingertat, and G. VINCENT CALDER* C. Leung, J. Phys. Chem., 75,2976 (1971).
RECEIVED OCTOBER 25, 1971 The Journal of Physical Chemistry, Vol. 76, No. 3, 197d
(2) W. D. Bascom, E. J. Brooks, and B. N. Worthington, Nature (London), 228, 1290 (1970). (3) E. R. Lippinoott, R. R. Stromberg, W. H. Grant, and G. L. Cessao, Science, 169, 1482.(1969).
