Acetylene vibrational frequency

Using a mechanical model and a set of force constants, Popov and Lubuzh (66ZPS498) have calculated vibration frequencies for polyacetylenic groups. But these calculations are rather complex and the data on the IR spectra of acetylenic... 

Photolysis of H3NBH3 with 121.5 nm radiation yields imidoborane, HBNH, which has been of theoretical interest Spectral shifts observed for several isotopic species containing °B, N, and D show clearly that the spectrum is due to HNBH which is isoelectronic with HBO, HCN and HCCH. From the spectrum of the isolated species two of the and one of the tr-type vibration frequencies for a linear molecule have been obtained. The location of the missing S (B-H stretch) frequency has been calculated. A comparison of observed and calculated frequencies for HBNH is given in Table 7. Another isolated product observed in these experiments is identified as HNB. This radical may be generated by photodissociation of HNBH subsequent to its formation. In this respect the photolysis mechanism would be similar to the formation of C2H from acetylene. 

The ELS spectra for these two hydrocarbon fragments are shown in Figure 8. Assignment of the observed vibrational frequencies is discussed in detail by Demuth and Ibach for the decomposition of acetylene on Ni(lll) (104). It is possible that species such as these are important surface intermediates under high pressure catalytic conditions (105, 106). Further studies in this area are in progress. 

Laser-ablated La atoms were codeposited at 4 K with acetylene in excess Ar. The products La(C2H2), LaCCH2, HLaCCH, and La2(C2H2), were all characterized using IR spectroscopy. DFT calculations gave calculated vibrational frequencies, relative absorption intensities, and isotopic shifts that supported the identification of these molecules from the matrix IR... 

Force Constants of Acetylene. From the vibrational frequencies of the normal modes, one can calculate the force constants for the different bond stretches and angle bends in the C2H2 molecule. In the most complete valence-bond, harmonic-oscillator approximation, the potential energy for C2H2 can be written as ... 

When C2D2 frequencies are used, in should be replaced by /Hq. The force constants for acetylene can be calculated from these relations using the measured vibrational frequencies, and the bond lengths can be determined from the rotational analysis described below. If one expresses the frequencies in cm units and the masses in appropriate isotopic mass units, the factors 4t7 should be replaced by 4t7 c 10 NY = 5.8918 X 10 (this includes a factor of 10 kg/g mass conversion). This substitution gives the force constants Y, kg, and Yr in N m units and the bending constants kg and kgg in units of N m. 

Free C2H2 molecule. The free acetylene molecule was optimized at the BLYP/6-31G level. Then, vibrational frequencies for the optimised free C2H2 molecule have been calculated. The computed geometry, C-C Mulliken overlap population and vibrational frequencies are listed in Table 2 along with the corresponding experimental values [41, 42], Both of these results show that the acetylene molecule in its ground state is linear with a Dmi, symmetry. 

The difference in stability between the diagonal fourfold-hollow site and the aligned fourfold-hollow site is only 9.2 kJ.moT. This difference is too small to allow us to conclude unambiguously which is the most stable adsorption mode. This served as a motivation to the calculation of the vibrational frequencies for acetylene adsorbed on these two most stable adsorption sites. The results obtained, along with the available experimental results, are presented in Table 4. 

In the present paper we have presented some studies concerning the adsorption of acetylene on copper (100) surfaces, and the adsorption of ethylene on the (100) surfaces of nickel, palladium and platinum. In all these studies we have used a cluster with the same shape and size. Despite the limited size of the clusters used, some very interesting features of the systems adsorbate - metal surfaces were determined, namely adsorbate geometries, adsorption energies and vibrational frequencies. 

Resonant raman spectroscopy has proved to be another valuable tool for the study of the structure of the polydiacetylene chain. Due to the resonance enhancement the spectra are compared to greatly simplified, infrared spectra and show as principle feature only the in-plane modes of the polymer chain. The correlation of the CsC and C = C stretching modes and their temperature dependence have been interpreted as resonances between the mesomeric structures (I) and (II) i32) Hoy(rever, a model using simple anharmonic force constants for the acetylene structure (II) is in good agreement with the experiment, e.g, the temperature and pressure dependence of the vibration frequency and the mechanical properties... 

The vibrational frequencies, bond angle, and configuration were estimated by Chupka, et al (2). Bond distances were calculated assuming the C-C bond distance was the same as in acetylene. 

Sellers H (1990) Structures and vibrational frequencies of acetylene in 3 binding sites on the Pd(lll) surface. J Phys Chem 94 8329... 

Figure 9.20 reveals the mechanistic reason for the profound changes in the intramolecular dynamics in acetylene that occur near Nb = 10 (Ka = 3). The opposite sign anharmonicities of the cis- and trans-bending vibrations cause the effective vibrational frequencies for the cis and trans normal mode benders, ui + 2vx, to become equal near Ubend = 10,... 

The monoacetylide derivatives are also important examples of simple organometallic molecules. The reactions of excited Ca and Sr with acetylene in a Broida oven resulted in the detection of the CaCCH and SrCCH molecules [127], Table 8 compares the observed and calculated X2E 1 vibrational frequencies. This work was rapidly followed by a rotational analysis of a vibrational band in the A2U X2E+ transition of CaCCH [128], It was assumed that we had found the 0-0 band, but the low-resolution molecular beam experiment of Whitham et al. [39] showed that we had analyzed a hot band. 

The calculated harmonic vibrational frequencies for N2O are shown in Table 23. The derived experimental values are also quoted ". As expected, the MP2 N—N (2159 cm ) and N—O (1251 cm ) stretches are underestimated, with the triple-bond harmonic mode having the larger calculated-experiment gap. The observed fundamental vibrational frequencies(2224 and 1285 cm respectively) for these two modes are naturally smaller than the derived harmonic values, and are therefore closer to the calculated stretch frequencies. The bending mode is very well calculated, despite the absence of f-type functions from the basis set, as has been shown necessary for acetylene. 

Waals complexes. One is that the vibrational frequencies of each monomer change little upon dimer formation. A similar statement holds for the bond distances and angles. Second, there is often a considerable gap between the frequencies of the monomer units and the van der Waals frequencies. Finally, it is interesting to compare the C—H frequencies and their shifts in the linear and T-shaped dimers. In the linear isomer, the C—H frequency of the HCN unit is free, while the acetylene C—H unit is bound as it forms part of the van der Waals bond. This difference in the C—H environment is reflected in the frequency shifts. That is, the HCN frequency is shifted by only 1 cm, while the bound acetylene frequency is shifted by 27 cm. In the T-shaped isomer, it is the other way around in that the acetylene C—H stretch is free while the HCN stretch is bound. (Apparently, the acetylene C—H stretch is not totally free in the T-shaped dimer as its shift is still 10 cm . )... 

Coordination of an acetylene to a transition metal makes the acetylene stretching frequency infrared "allowed" and shifts the vibration to frequencies that are 150 to 450 cm below that of the Raman band of the free alkyne. Larger changes in the vibrational frequency wpuld be expected when the complex adopts the structure of a "metallacyclopropene" instead of a coordinated alkyne, but this prediction has not been carefully explored. Like alkene complexes, alk3me complexes can possess rotational barriers about the metal-acetylene axis, and these rotational barriers depend on the symmetry of the orbitals of the metal fragment. 

The bands due to metal-acetylene vibrations V2 and V3 in complexes have not yet been identified because of a strong interaction between them. The frequency Vj is due to the C = C stretching vibration. The position of v(C = C) bands of coordinated acetylenes is influenced by the strength of tc back-bonding. The stronger the metal-acetylene interaction, the greater the lowering of the v(C = C) frequency of bonded acetylene compared to the free acetylene molecule (Table 6.20). 

We will now consolidate some of the ideas introduced in this chapter by deriving the vibrational frequencies of the linear acetylene molecule C2H2, in... 

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