ν5 Vibration of Adsorbed EthyleneL. H. Little Citation: The Journal of Chemical Physics 34, 342 (1961); doi: 10.1063/1.1731602 View online: http://dx.doi.org/10.1063/1.1731602 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/34/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Unimolecular reactions near threshold: The overtone vibration initiated decomposition of HOOH (5νOH) J. Chem. Phys. 84, 1508 (1986); 10.1063/1.450496 Intermolecular Raman intensity transfer in the ν (CO) vibrations of Mn(CO)5Br and Re(CO)5Br J. Chem. Phys. 72, 2131 (1980); 10.1063/1.439308 Frequency Shifts and Band Contours of Vibrational Transitions of Vapors Trapped in HighDensityPolyethylene. The ν3, ν5, ν6, ν5−ν3, and ν2−ν6 Bands of CHCl3 and the ν2, ν3 Modes of CS2 J. Chem. Phys. 42, 694 (1965); 10.1063/1.1695992 On the Vibrational Raman Spectrum of Gaseous Ethylene J. Chem. Phys. 21, 755 (1953); 10.1063/1.1699012 The Vibrational Frequencies of Ethylene J. Chem. Phys. 18, 118 (1950); 10.1063/1.1747428

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342 LETTERS TO THE EDITOR

Our results, together with the work of Nolle and Mahendroo,s imply that the pressure dependence of TI in at least most of the liquids studied by Benedek and Purcell6 should be interpreted as the pressure de­pendence of the impurity relaxation rather than the pressure dependence of the intrinsic relaxation of the pure liquids.

The nuclear spin-lattice relaxation time, TI , of the protons of these samples was measured at 30 Me by a pulse technique similar to that of Billings and Nolle7

within an accuracy of about 5%, limited chiefly by temperature control. Studies of the temperature de­pendence of these samples are in progress and will be reported shortly.

We wish to thank J. F. Harrod for the distilled water sample, and H. S. Sandhu for the use of his vacuum system.

* Supported in part by the National Research Council of Canada and the U. S. National Science Foundation.

t Holder of National Science Foundation Science Faculty Fellowship on leave of absence from the University of Wyoming.

1 G. Chiarotti, G. Cristiani, and L. Giulotto, Nuovo cimento 1, 863 (1955).

2 G. W. Nederbragt and C. A. Reilly, J. Chern. Phys. 24, 1110 (1956) .

3 A. W. Nolle and P. P. Mahendroo, J. Chern. Phys. 33, 863 (1960) .

4 J. H. Simpson and H. Y. Carr, Phys. Rev. 111, 1201 (1958). 6 H. S. Sandhu, J. Lees, and M. Bloom, Can. J. Chern. 38, 493

(1960) . 6 G. D. Benedek and E. M. Purcell, J. Chern. Phys. 22, 2003

(1954) . 7 J. J. Billings and A. W. Nolle, J. Chern. Phys. 29, 214 (1958).

Ps Vibration of Adsorbed Ethylene*

L. H. LITTLEt

Division of Applied Chemistry, National Research C01tncil, Ottawa, Canada

(Received September 12, 1960)

THE infrared-inactive Ps, C-H stretching vibra­tion of ethylene, has been variously assigned to

lines occurring at 3272 em-II and at 3075 cm-I 2 in the Raman spectrum of liquid ethylene. More recently, a detailed study of the infrared spectra of deuterated ethylenes led Crawford et at.,s to assign the Ps vibration at 3075 em-I. In the present investigation of the spectrum of ethylene adsorbed at 79°K on porous Vycor glass, evidence is given to support the latter assignment.

Sheppard and Yates4 have shown that bands which normally occur only in the Raman spectrum, may appear in the infrared spectrum of adsorbed molecules. These vibrations are infrared-forbidden, but appear in the spectrum of the adsorbed material because of

20

z 2 40 l-e.. 0:: o Cf)

~ 60

80 3093

FIG. 1. Infrared spectrum of ethylene adsorbed at 79°K on porous Vycor glass.

perturbation of the molecules by the surface, and loss of molecular symmetry.

The spectrum of ethylene (Fig. 1) adsorbed on porous glass, shows bands at 3093 cm-I and 2977 em-I, due to infrared-allowed P9 and Pn, C-H stretching vibrations, respectively. In addition to these, a band appears at 3007 em-I, which has been assigned by Sheppard and Yates4 to the PI, totally symmetric, C-H stretching mode. This band appears in the infrared spectrum due to loss of symmetry of the ethylene molecule in the adsorbed state. In the spec­trum (Fig. 1), measured with somewhat higher resolu­tion, a fourth band appears at 3070 em-I.

Herzbergl has assigned a weak band at 3075 cm-l in the Raman spectrum of liquid ethylene, to the P9 (3105 cm-l in the infrared spectrum of the gas phase) vibra­tion. This band is Raman-forbidden but was said to appear,t displaced to lower frequency, because of molecular interaction in the liquid state.

Unless there is more than one type of adsorbed ethylene, the infrared band, appearing at 3070 cm-l

in Fig. 1, cannot be assigned to the P9 vibration since an intense band occurs at 3093 cm-l due to this vibra­tion. In view of this fact, it is probable that the 3070 cm-l band is due to the Ps vibration since no combina­tion of other bands can explain its occurrence.

The relative intensities of the 3070 cm-l and 3093 cm-1 bands appear to be independent of surface cover­age, suggesting that only one surface species was in­volved. Both bands were completely removed from the spectrum of the surface by room temperature evacua­tion of the sample, mass spectrometric analysis of the material so obtained, showed it to be pure ethylene.

The assignment of the Ps vibration to 3070 em-I, is supported by the observation that no band appeared in the spectrum of adsorbed ethylene at 3272 em-I, the frequency assigned by Herzberg to this vibration. How-

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LETTERS TO THE EDITOR 343

ever, this fact alone would not rule out the assignment of the Jls vibration to 3272 cm-1 since there is no a priori reason why an infrared-forbidden band of surface molecules, should appear with measurable intensity in the spectrum.

* Issued as National Research Council No. 6093. t National Research Council, Postdoctorate Fellow 1958--1960. 1 G. Herzberg, Infrared and Raman Spectra (D. Van Nostrand

Company, Inc., Princeton, New Jersey, 1945), p. 327. 2 For details see work cited in footnote 1. 3 B. L. Crawford, J. E. Lancaster and R. G. Inskeep, J. Chern.

Phys. 21, 678 (1953). 4 N. Sheppard and D. J. C. Yates, Proc. Roy. Soc. (London)

A238, 69 (1956).

Note on the Sublimation of Ammonium Perchlorate

H. M. CASSEL AND 1. LIEBMAN

Explosives Research Laboratory, Bureau of Mines, Pittsburgh, Pennsylvania

(Received May 6, 1960)

AM~ON~U.M salts of weaker acids sublime through 1"1. dIssocIatIOn, e.g., NH4Cl=NHa+HCl. Thus far, however, the mechanism by which ammonium per­chlorate (APC) sublimes, has not been definitely established experimentally. Galway and Jacobs,1 in a recent study of the high-temperature decomposition of APC, express the opinion that APC probably sublimes undissociated, owing to the powerful proton-donating power of perchloric acid, and to the stabilization by hydrogen bonding. An occasional observation by Freeman and Anderson2 of spheroidal crystallites3

formed in the sublimation under vacuum of APC, sug­gested that in this process, precipitation of a liquid phase precedes crystallization.

In an attempt to verify this expectation, a 3/4-in. test tube, 6 in. long, containing about 2S0 mg pure APC, was heated at the base to 300°C and then evac­uated. Temperatures of the glass wall were measured by thermocouples attached at the base and at 1-, 2-, and 3-in. distances. Within a few minutes after closing the outlet to the pump, a deposit of droplets appeared on the glass where the temperature had reached SO°C. On repeating evacuation, these drops coalesced to form larger patches of liquid, which extended to cooler parts of the wall. It was first thought that liquid APC had been formed. However, when pure ammonia was ad­mitted, the liquid drops immediately crystallized. Infrared analysis showed 62% HNOa and 12±S% HCI04; in addition, the presence of HCI was confirmed by a "wet test." This agrees with a measured pH of 3.3 for an aqueous solution of the droplets. The HCI04/

HNOa ratio as determined, suggests the decomposition reaction:

NH4Cl04= (HCI04+NHa)/6

+S (HNOa+ HCI+ H20) /6.

Accordingly, only one-sixth of the reaction would be simple dissociation. The heat generated upon addition of ammonia is more than 60 kcal and is thus sufficient to evaporate the excess of water and to release a crystal­line product, as described.

1 A. K. Galway and P. W. M. Jacobs, J. Chern. Soc. (London) 1959,837.

2 Information obtained verbally from E. S. Freeman and D. Anderson, The Feltman Research Laboratory, Picatinny Arsenal, Dover, New Jersey.

3 E. D. Jones, D. S. Burgess, and E. S. Amis, Z. physik. Chern. 4, 222 (1955).

Ion-N eutral Reactions in the Helium-Hydrogen System

MARTIN HERTZBERG, DONALD RAPP, IRENE B. ORTENBURGER, AND DONALD D. BRIGLIA

Lockheed Missiles and Space Division, Palo Alto, California

(Received August 22, 1960)

WE have measured the appearance potential of the HeH+ ion, formed by primary ions with average

kinetic energy above 10 ev, and have found that it coincides with the appearance potential of the H2+ ion. As a result, all three secondary reactions listed below must be considered in the helium-hydrogen system:

H2++ H2----7Ha++ H

He++H2----7HeH++H

H2++ He----7HeH++ H,

( 1)

(2)

(3)

and it is not permissible to neglect reaction (3) a priori, as has been done in obtaining reaction cross sections from the observed secondary to primary current ratios.l

Below the appearance potential of the helium ion, only reactions (1) and (3) need be considered, and the ratio of reaction cross sections Qa/Ql was measured to be 0.07 ±0.03. An analysis of the data in the region above the appearance potential of helium, indicates that reaction (2) proceeds more rapidly than reaction (3) by about a factor of two. These measurements are con­clusive proof of the presence of reaction (3) for which evidence has already been obtained in a gaseous dis­charge study.2

If a value of 1.8 ev is assumeda for the dissociation energy of the HeH+ ion into He and H+, then reaction (2) is 8.3-ev exoenergetic, and reaction (3) is 0.8-ev

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