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J. Phys. Chem. 1982, 86, 442-443
Determination of Energies of the Quasifree Electron State V , in Solid Cycloparaffins from Electron Transmission Spectra Kenzo Hiraoka’ and Masajl Nara Faculty of Engineering, Yamanashi University, Takeda-4, Kofu 400, Japan (Received: November 12, 198 1)
The transmission of low-energy electrons (0-15 eV) through 10-100-A films of organic compounds deposited on a stainless steel metal block at -80 K has been systematically studied. Structures are clearly indicated by electron current It transmitted through a thin film as a function of the incident electron energy Vi, displayed as dIt/dVi w. V> With increasing film thickneas, a drastic decrease in the height of the f i t peak (due to injection of electrons into the film) and the appearance and growth of a second peak are observed for alkanes, alkenes, alcohols, and ethers. The energies of a quasi-freeelectron state, Vo,for these compounds have been determined by measuring the energy of the second peak relative to the first peak of the electron transmission spectra (dIt/dVi vs. Vi). In these studies, it was found that for normal alkanes the Vo value shows an increase from 0.6 eV for pentane to 0.9 eV for hexane, but the increase in carbon number from heptane to decane has little effect on the Vo values. It is further found that cyclic hydrocarbons have smaller V$ than linear ones, e.g., for cyclohexane Vo = 0.5 eV and for hexane Vo = 0.9 eV.
In this work, the transmission spectra for a series of cycloparaffins were measured and the dependence of the carbon number on the Vo values were investigated. In Figure 1 are shown the transmission spectra of 10-langmuir4 thick samples of cyclopentane, cyclohexane, cycloheptane, cyclooctane, and cyclododecane. With increasing sample deposition, a drastic decrease of the first peak and the appearance of a second peak are observed for C5-* cycloparaffins, i.e., these compounds have positive Vo values. By measuring the energy of the second peak relative to the first peak? the Vovalues for cyclopentane, cyclohexane, cycloheptane, and cyclooctane are determined to be 0.3,0.5,0.4,and 0.3 eV, respectively. The Vovalue shows an increase from 0.0 eV for cyclopentane to 0.5 eV for cyclohexane, but it shows a decrease as the carbon number increases from cyclohexane (0.5 eV) to cyclooctane (0.3 eV). The height of the first peak of the transmission spectra for cyclododecane shows only a slight decrease with the deposition of a 2-langmuir thick sample. With an increase in the film thickness from 2 to 10 langmuirs, a slight increase in the width of the first peak was observed, but the second peak did not appear (see Figure 1). These results indicate that the energy of the quasi-free electron state Vofor solid cyclododecane is close to the vacuum level. Due to the poor resolution of the present experimental system, the second peak may not appear for a compound whose Vo is between 0 and 0.2 eV. Thus the V, value for cyclododecane may only be estimated as 10.2 eV. The observed decrease of the Vo values for cycloparaffins C,H2,, for n I 5 is quite a contrast to the case of normal paraffins for which an increase of the Vo values is observed with an increase in carbon number. These experimental results provide further evidence of the marked effect of the molecular structure on the Vo values. The transmission spectra may correspond approximately to the first derivative of the distribution function of density-of-states of the conduction band.3 In the transmission spectra of CGecycloparaffins, a small but sharp peak at 2 eV is observed. As the profile of the conduction band
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(1) K.Hiraoka and M. Nara, Bull. Chem. SOC. Jpn., 54,1589(1981). (2)K.Hiraoka and M. Nara, Bull. Chem. SOC.Jpn., 54,2068(1981). (3)K.Hiraoka, J. Phys. Chem., 86,4008 (1981). (4)The amounts of the gas admitted in the vacuum chamber is expressed in langmuir units (1 langmuir = 1 X IO” torr a). 0022-3654/82/2086-0442$0 1.25/0
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cyclooctane
> -0
\
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cyclopentane
I!
I\ 0
metal
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4
block
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Figure 1. The transmission spectra dI,/dVI vs. VI for cyclopentane, cyclohexane, cycloheptane, cyclooctane, cyclododecane, and metal block. The film thickness for each sample is 10 langmuirs.
must be reflected in the transmission spectra at least to some extent, this peak might correspond to the rising part of the high density-of-states of the conduction band. It is worthwhile to note that such a peak does not appear in the transmission spectra of cyclopentane and cyclododecane. This difference in the low-energy events may be due to the difference in the structure of each solid sample. Recently, Ueno et aL5studied the kinetic energy (5) N. Ueno, K. Sugita, and S. Kiyono, Chem. Phys. Lett., 82, 296 (1981).
0 1982 American Chemical Society
J. Phys. Chem. 1982, 86, 443-447
spectra of secondary electrons emitted from n-C4Hw thin film. They observed that the spectral features associated with the high density-of-states regions of the conduction bands of crystalline n-C4Hw are smeared out sharply at the bulk melting point, Le., the conduction band features are only observed for crystalline samples. Their results suggest that the cyclopentane and cyclododecane samples deposited on the metal surface in situ under vacuum at -80 K glassy to some extent. A more detailed investigation of the conduction band structure of alkanes, alkenes, and aromatics and of the dynamic processes of molecules in the films of these compounds is now in progress.6 Another characteristic difference of the transmission spectra between C5,12and C, cycloparaffmsis that a broad negative peak appearing at -14 eV for cyclopentane is considerably smaller than those for CW cycloparaffms and is almost missing of cyclododecane. This broad negative peak is observed for all compounds of alkanes, alkenes, ethers, alcohols, and ketones in our experiments with the exception of only two compounds. One exception is ice2
(6) K. Hiraoka and M. Nara, to be submitted for publication in J. Phys. Chem.
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and the other is cyclododecane. The transmission spectra of these compounds are similar in that they are markedly structureless compared to those of other compounds. This may be owing to the more or less glassy structure of films of these two compounds. Because the broad negative peak is due to the ionization of molecules in a film,2i3 the scattering processes of incident and ejected electrons in crystalline solids must be quite different from those in glassy solids. When the reflection coefficient of electrons at the film-vacuum interface ( R ) is small, a discernible negative peak is expected in the transmission spectra. In other words, the larger the reflection coefficient is, the less prominent the negative peak becomes. The increase of the value of R in glassy solids may be due to the efficient energy loss of electrons in the solids, Le., the mean free path of electron-phonon scattering is much shorter in glassy solids than in crystalline solids. In the case of V,, I 0, after an electron cascades to the bottom of the conduction band by losing its kinetic energy by a severe electron-phonon scattering in glassy solid, it can hardly escape to the vacuum due to the energy barrier (-Vo) and to the surface potential of the solida3This may account for the absence of a broad negative peak in the transmission spectra of cyclododecane and ice.
ARTICLES Group Additivity Parameters for the Estimation of Thermochemical Properties of Gaseous and Liquid Nltrlles James
Y. Chu, Tam T. Nguyen, and Kelth D. Klng"
Department of Chemical Engineering, University of Adale&, Adele&, In F h d FOrm: JU/y 27, 1981)
South Austral& 5001 (Received: April 2, 1981;
Contributions of the CN-containing groups in.the estimation of thermochemical properties (AH?,So,and Cpo) of gaseous and liquid nitriles according to the group additivity method are assessed. Taking into account the more recent data, existing group values are revised and some new group contributions are evaluated for the gas phase. Group contributions are derived for the fist time for these compounds in the liquid state. In general, the data are quite self-consistent, but further measurements of AHf0 are needed for a better evaluation of significant next-to-nearest-neighborinteractions in unsaturated nitriles and polycyano compounds. Introduction A group additivity scheme for the estimation of thermochemical properties (AH,',So, C ") of compounds in the ideal gas state was proposed by benson and Buss' in 0022-3654/82/2086-0443$01.25/0
1958. Some 10 years later the procedure was extended and applied to a large variety of molecules by Benson and (1) Benson, S. W.; Buss, J. H.J. Chem. Phys. 1968,29, 546.
0 1982 American Chemical Society
