Vibrational spectrum of ethynol: AB initio calculation and matrix isolation studies

Chemical Physics 117 (1987) 65-72 North-Holland, Amsterdam

VIBRATIONAL SPECTRUM OF ETHYNOL: AB INITIO CALCULATION AND MATRIX ISOLATION Josef DOMMEN, Laborarmy

Martin RODLER

STUDIES

and Tae-Kyu HA’

for Physical Chemistty, Swiss Federal Institute of Technologv Universitiitstrasse

22, CH-8092 Zurich, Switzerland

Received 4 May 1987

Ab initio calculation of the harmonic force field of ethynol have been performed. A 6-31G * * basis set was used and electron correlation was taken into account by applying perturbation theory carried to second order (MP2). After application of experimental scaling factors, vibrational frequencies have been computed. From numerical differentiation of the dipole moments, intensities of the vibrational transitions have been derived. With the help of the harmonic force field the quartic centrifugal distortion constants have been obtained. The reactions OH + CH,CzCH, OH + HC,H, H + C,O, and Ar * + HCXCOOH have been tried to produce ethynol. The reaction products were frozen in an argon matrix and their IR spectrum recorded. Many new absorption lines appeared but there was no evidence for ethynol.

1. Introduction Ethynol (hydroxy acetylene, H--OH) is one of the less stable isomers of ketene (H,C=C=G). Besides the chemist’s interest in this molecule due to the structure, it also attracts astrophysicists. Unstable tautomers of well-known species can be present in detectable quantities in interstellar molecular clouds, provided the barrier to unimolecular rearrangement is sufficiently high. The classical example is isocyanic acid (HNC) which can be found in considerable amounts besides the more abundant isomer cyanic acid (HCN) [1,2]. The two species are not in thermal equilibrium in molecular clouds. The presence or absence of unstable isomers provide valuable information about the chemical processes occurring in these regions. Extensive ab initio calculations of Tanaka and Yoshime [3] yielded an energy difference of 150.9 kJ/mol between ethynol and ketene. They also give a barrier of 306.4 kJ/mol for the unimolecular hydrogen shift from ethynol to ketene. This indicates that the former should be a detectable species. Indeed, recently van Baar et al. [4] for the first time detected it in a tandem mass spectrome-

’ To whom correspondence

should be addressed.

ter by decarbonylation of propiolic acid. In the past ethynol has also been postulated in reactions of hydroxyl radicals with propyne [5] and of hydrogen atoms with carbon suboxide [6]. Matrix isolation infrared spectroscopy has proven an invaluable tool for the immobilization and characterisation of reactive and unstable molecules. An analogous molecule which has been studied by this method is vinyl alcohol [7]. It is the less stable tautomer of acetaldehyde. Therefore, we applied this technique in the search for ethynol. Several radical reactions have been tried to produce this species: OH + CH,WH, OH + HC,H, and H + C,O,. A further attempt was the reaction of excited argon atoms with propiolic acid. Many new absorption lines have been recorded but no evidence for ethynol could be found. Since most of these reactions yielded a very complex mixture of products, accurate predictions of the vibrational frequencies and the intensities of the fundamental bands were essential. Thus the experimental efforts were paralleled by theoretical calculations. The only ab initio computation of infrared frequencies found in the literature originates from DeFrees and McLean [8]. They calculated the force constants at the HF/3-21G level and scaled them in an attempt to compensate for the systematic overestimation of force con-

0301-0104/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

J. Dommen et al. / Vibrational spectrum of ethynol

66

stants at that level of theory. No intensities were given. Because the theoretical level as well as the specific scaling procedure applied did ‘not seem sufficiently reliable for the interpretation of the experimental results, we recalculate the harmonic force field at the MP2/6-31G * * level. A scaling procedure was employed which assigns individual scaling factors to each geometrical parameter. In addition, the intensities of the fundamental bands were calculated at the HF/6-31G * * level.

2. Computational

details

The quantum-chemical calculations were performed with the GAUSSIAN 80 and GAUSSIAN 82 system of programs [9,10]. The 6-31G* * basis set [ll] was used throughout. This split valence basis set includes p-type polarization functions for hydrogens as well as d-type functions for carbon and oxygen. For the evaluation of the force .constant matrix, electron correlation was taken into account by applying Moller-Plesset perturbation theory carried to second order (MP2) [12]. At this level of theory GAUSSIAN 82 allows the analytical determination of forces. The force constants were obtained by numerical differentiation of the forces whereby the geometrical parameters were changed by 0.005 bohr or 0.005 rad. In order to calculate the intensities of the infrared bands the derivative of the dipole moment components with respect to the structural parameters had to be computed. This was numerically done at the Hartree-Fock (HF) level. Only recently higherlevel analytical calculations of intensities became available but were just applied to small molecules because of the large computation power necessary. For the calculation of vibrational frequencies and centrifugal distortion constants the program ASYM 20 [13] was used.

3. Computational

chosen. Pulay et al. [14] recommended the adoption of the best available structure as a reference geometry rather than the optimized geometry from the level of theory employed in the force field calculations. Thus the structure resulting from a CISD + Q/DZ + P calculation (fig. 1) [8] was used as reference geometry for the computations. It shows only minor differences with the results from a MP3/6-31G * * calculation [15]. Because of the C, symmetry of ethynol the normal modes can be assigned to two different symmetry species (A’ und A”). Seven fundamental modes represent in-plane vibrations, whereas the number of out-of-plane fundamental modes is two. Let us consider A’ block first. The symmetry coordinates chosen represent simple bond stretches and angle bendings. They are defined in table 1. The harmonic force constant matrix computed at the MP2 level of theory is listed in table 2. In order to assess the influence of the electron correlation on the force constants an analogous calculation was carried out at the HF level. The most noticeable differences concern the CC stretching and the bending diagonal force constants. They all are larger at the HF level. For the three bending force constants the MP2 values are on the average smaller by 22%. The structure of ethynol is somewhat unusual. Since CC0 and CCH angles are very near to 180 “, four nuclei lie virtually on a straight line. This makes the adoption of torsional symmetry coordinates in the A” block inappropriate because some elements of the G matrix become enormously large. Therefore, setting the two angles to 180 O, the out-of-plane coordinates could be defined as linear bendings. In this way all resulting G-matrix elements become reasonable. Corresponding modifications were also made for the calculations of the force constants.

results and discussion

3.1. The vibrational spectrum of ethynol Due to anharmonicity, the theoretical force constants depend significantly on the geometry

Fig. 1. Reference geometry and atomic numbexing scheme used for the calculation of the harmonic force field.

J. Dommen et al. / Vibrational spectrum of ethynol Table 1 Definition of the symmetry coordinates for ethynol a) A’ block

A” block

SoH= rW-4 1 SC0= rG0) See= rGC2) SC, = GHd sCOH = 4WW SC,, = aW,C,Q SC,, = 4C,‘%H)

sm=

Y(C,C,O) b,

&BT= YG’~%)

W

‘) Atomic numbering scheme in fig. 1. b, Linear bendings.

It is well known that harmonic force constants calculated at the HF level are systematically too large. To a lesser extent this also applies to MP2 calculations. Scaling factors have been introduced to compensate this effect [14,16-181. They have been adapted in such a way that they do not only account for the deficiencies of the theoretical treatment (insufficient basis set and incomplete consideration of electron correlation) but also allow for the anharmonic contributions to the observed vibrational transitions. DeFrees and McLean [19] extracted a uniform scaling factor comparing MP2/6-31G * calculations with experimentally measured frequencies of a large set of mole-

Table 2 Calculated force constant matrix for ethynol

61

cules. Because a specific scaling factor for each normal mode was considered more appropriate to reliably predict infrared frequencies, we were forced to derive our own set of scaling factors. Methanol and acetylene have been chosen as reference compounds since much infrared data are available for these molecules and they have some structural features in comment with ethynol. Analogous MP2 calculations as for ethynol have been performed for them. The theoretical force constants have then been scaled to reach maximum agreement between calculated and observed fundamental frequencies for all the isotopic species found in the literature [20-231. The obtained scaling factors have then been transferred to ethynol: from methanol: OH: 0.88, CO: 0.91, COH: 0.94; from acetylene: CC: 0.91, CH: 0.88, CCH: 0.92. The scaling factors for the bending force constants are much closer to 1.0 than the corresponding HF values. No well studied molecule with a similar C=C-0 moiety as in ethynol could be found in the literature. Therefore, a scaling factor of 0.90 has been assumed for the CC0 bending coordinate. The same scaling factors have been applied to the two out-of-plane bending coordinates as for their corresponding in-plane coordinates. For larger molecules it is still not feasible to derive scale factors for all of the force constants.

‘) (unscaled)

A’ block

so, SC0 See sCH

SCOH Scc0 SCCH

SO”

SC0

SC,

8.289 0.009 -0.100 0.000 0.193 - 0.033 -0.017

8.168 0.164 - 0.043 0.571 - 0.040 - 0.012

-

G33

%3-i

16.389 0.076 0.005 0.006 0.001

SCH

SCOH

&co

SCCH

6.574 0.003 0.000 -0.001

0.777 - 0.067 - 0.024

0.405 0.158

0.205

A“ block

s, SER

0.474 -0.167

0.149

‘) Values from MP2/6-31G * * calculations done at the CISD + Q/DZ + P geometry [S]. Stretching force constants are in mdyn/A (aJ/A2), bending force constants in mdyn A/rad2 (aJ/rad2).

68

J. Dommen et al. / Vibrational spectrum of ethynol

Therefore the calculated MP2 force constant matrix given in table 2 has been scaled accordingly to the procedure of Pulay et al. [14]: The diagonal constants have been multiplied with the scaling factors given above whereas the off-diagonal elements were scaled with the geometrical means of the scaling factors of the two corresponding diagonal constants. In this way the number of independent scaling factors is reduced drastically. The resulting scaled ab initio force field led to the infrared transition frequencies given in table 3. The table also lists the calculated intensities along with the frequencies obtained by DeFrees and McLean [8] from a scaled HF/3-21G force field. In their approach all frequencies have been uniformly scaled by 0.89. The largest differences are found in the region below lOCKIcm-‘. At the MP2 level all these frequencies are lower by up to 280 cm -l. The frequencies above 1000 cm-’ are in good agreement. Only the OH stretching frequencies differs by nearly 200 cm-‘. Besides the rather unspecific scaling procedure adopted in ref: [8], the different quantum-chemical approaches appear to be the major reason for the discrepancies. DeFrees and McLean [19] calculated a mean error of 20 cm-’ when correcting theoretical MP2 harmonic frequencies mode-by-mode. Because of the unusual structure of ethynol the mean error of our predicted vibrational frequencies may be higher. Table 3 Calculated vibrational frequencies (in cm-‘) from scaled harmonic force field and intensities (in km/mol) of ethynol Mode

This work

(symmetry)

frequency

intensity a)

3613 3295 2153 1205 1025 568 348 502 350

153 99 125 1 52 50 2 47 29

“1

(A’)

“2

(A’)

“3

(A’)

“4 (A’) ys (A’) “6 (A’) “7

(A’)

“6

(A”)

“9

(A”)

Ref. [8] frequency 3418 3285 2206 1217 1003 841 473 773 517

‘) Integrated absorption coefficient A, = (l/CL)/ ln(I,,/ I) dG, where C is concentration (mol l-l), L is optical path length (cm) and I,, and I, respectively, are the intensities of the incident and transmitted light.

Table 4 Calculated effective rotational constants (in MHz) and quartic centrifugal distortion constants ‘) (in kHz) of ethynol and ethynol-OD

Ao go CO

OJ DJK DK

d, da

HC=C-OH

HGC-OD

706200 b, 9678 b, 9525 b, 3.3 560 250@@0 - 0.044 - 0.064

379400 b, 9271 b, 9024 b, 3.0 310 1OOOOO - 0.092 -0.018

‘) The Sreduction of the I’ representation was applied [24]. b, From ref. [8]; note the wrong (inverse definition of the scaling factor c. Calculated effective rotational constants forHCCOH from ref. [15]: B, = 9695, Co = 9534.

3.2. The rotational spectrum of ethynol DeFrees and McLean [8] and Brown et al. [15] derived theoretical rotational constants from their optimized ab initio geometries. These constants were empirical corrected for deficiencies of the theoretical treatment and the difference between equilibrium and effective rotational. constants. These corrected rotational constants are at present the best available predictions for the ground-state rotational of ethynol. With the harmonic force field available it is possible to calculate the quartic centrifugal distortion constants as well. They are given in table 4. These data might facilitate the unequivocal assignment of a rotational spectrum to ethynol. As the constants for the OD species are markedly different, the data of this isotopic species are also included in table 4.

4. Experimental details A Pyrex flow tube 1.8 cm in diameter and 32 cm in length is coupled to a cryostat. Two concentric sliding injector tubes are introduced into the flow tube to admit NO2 (outer tube) and the reactant (inner tube). Typically, argon mixed with = 2% hydrogen comprises the main flow, which was passed through a microwave discharge before being introduced into the flow tube where the H

69

J. Dommen et al. / Vibrational spectrum of ethynol

atoms react with NO2 to NO and OH. The reaction mixture was sampled through a pinhole. Details of this configuration which was used for hydroxyl radical and hydrogen atom reactions, can be found elsewhere (setup A) [25]. For the excited argon studies a 8 mm inner diameter open ended Pyrex tube was used. The sample was added in front of the outlet of the tube (setup B). The sample was deposited on a CsI window maintained at 12-13 K. An Air Products model DE-202 closed-cycle helium refrigeration system was used. Infrared spectra (4000-200 cm-‘) were recorded with a Perk&Elmer 325 spectrophotometer with a resolution between 0.6 and 2 cm-‘. NO, (Matheson) was first mixed with an excess amount of oxygen over P,O, for at least 12 h at room temperature and then frozen out at liquid nitrogen temperature. The excess Oz was pumped off and the NO, was distilled. The purified NOz was mixed with argon in the ratio l/4 to l/25. CH,CCH (Matheson) was used without further purification. Diacetylene was prepared according to the method of Armitage et al. [26] and purified by trap-to-trap distillation. Propiolic acid (Fluka, pm-urn) was also purified by trap-to-trap distillation. Carbon suboxide was prepared by mixing phosphorous pentoxide and malonic acid in 10/l weight ratio. After evacuation it was heated to 100 QC. The evolving gas mixture tias led through traps cooled to 200 and 77 K. At 200 K acetic acid was frozen out. CO, and C,O, were collected at 77 K and separated by pumping off CO, at 150 K.

5. Experimental

results and discussion

5. I. OH + CH,CCH From the infrared spectrum of the reaction mixture the following products could be identiHCOOH, H&O, C,H,, fied: CO, CO,, H,CCCH,, H&CO, HCN and HNCO. Some additional weak absorption lines could not be assigned but they can be excluded as belonging to ethynol. In case of an excess of NO2 in the reaction mixture less allene and acetylene, but more of the oxygen containing compounds were

formed. From mass spectrometry, Kanofsky et al. [5] proposed the following reaction routes: OH + CH,C=CH

+ C,H,O

+ CH,

(1)

+ C,H,O

+ H

(2)

+ C,H,

+ H,O

(3)

+ C,H,

+ CH,O.

(4)

From the products observed by us, only C,H,O and H&O could be primary reaction products. Our results show that the structure of C,H,O is ketene rather than ethynol. A possible mechanism is a migration of the hydrogen atom of the hydroxyl group in the addition complex before the methyl group leaves

[HJZcH,J -,[H,ZL,] + H,C=C=O

+ CH,.

(5)

Kanofsky et al. [5] observed only minor amounts of formaldehyde. Under our experimental conditions it could well be a secondary reaction product similar to the other products observed. Various reaction routes could be formulated for them but this would be very speculative from our restricted data. 5.2. OH + H,C, This reaction has been tried hoping the reaction route PH OH + HCCCCH + [ HCCCCH] --* HCCOH + CCH

(6)

might be of some importance although the OH radical is expected to attack preferentially the end position. Since diacetylene is already quite a large molecule, the reaction mixture became very complex yielding a large number of infrared absorption lines. The method to produce OH radicals already gives rise to various products like HNO,, cis-HONO, trans-HONO, NO. Since diacetylene reacts only partially, its very strong absorption

J. Dommen et al. / Vibrational spectrum of ethynol

70

lines also cover a broad range of the spectrum. The following products could be identified: CO, CO,, HCOOH, H&O and C,H,. A large portion of absorption lines of weak and medium intensity could not be assigned. These problems severely complicated the analysis. The regions at 3300 and where strong absorption lines of 2150 cm-‘, ethynol are expected, are covered by broad lines of diacetylene and CO/CO * H,O. To search for ethynol we focused on the regions between 3400-3700 and 1000-1100 cm-‘, which were relatively free from strong absorption lines. At 3613 and 1025 cm-’ the OH- and CO-stretch frequencies were predicted. Frequencies observed in these regions are given in table 5. The absorptions observed at 3613 and 1069 cm-’ do not correlate as could be shown from comparisons of their intensities at different experimental conditions. The other potential CO-stretch frequencies are too weak to correlate them reliably with the OH-stretch line. Upon deuteration a frequency shift of - 2.3 cm-’ has been calculated for the CO-stretch. Variation of the CO/COH out-of-diagonal force constant by f 10% gave even positive shifts. This excludes the absorption lines at 1069 and 1042 cm-’ which show too large negative shifts. A higher pressure in the reactor increased the intensity of the remaining lines only slightly. Simultaneously, new lines appeared and the yield of HNO, and HONO grew rapidly. All these problems make this system very inconvenient for our purposes.

Table 5 Observed infrared absorption lines in the OH and CO stretching region of the reaction mixture OH(OD) + HC, H i; (cm-‘) OH

OD

3613

2671 2608 2598 1055 1035 1017 1008 1003 1000

1069 1042 1016 1008 1003 999

5.3. H + C,O, This reaction has been investigated extensively by Faubel and Wagner [6]. They proposed the following mechanism: C,O, + H --, HC,O + CO,

(7) (8) (9)

HC,O + H --, CH, + CO, HC,O+H+M+HC,OH+M, CH, + CH, + C,H, CH, + C,H, H&OH

+ 2H,

+ M --* C,H,

(10) + M,

+ H --, C,H + H,O,

01) 02)

HC,O + CH, + C,H + CH,O,

(13)

C,H + C,H,

04)

+ C,H,

+ H.

The main products found by mass spectrometry were CO, C,H,, C,H, and C,H,. As can be seen from reaction (9) ethynol was postulated as one of the products. In their spectra they cannot identify it because the crucial mass fragments m/e = 17 and 25 are hidden by the masses of H,O and C,H,. We tried two types of experiment. Using the open-ended tube, a mixture of argon and hydrogen was led through the discharge. In this configuration the reaction time is very short and only few collisions are possible. The products identified were CO, C,O, HO2 and H,O. Weak to moderately intense unassigned absorptions also appeared at 2144, 2130, 2096, 2087, 2019, 1790, 1787, 1530, 1414 and 1409 cm-‘. C,O has probably been produced by excited argon atoms. Jacox et al. [27] photolysed C,O, in an argon matrix observing C,O and several unassigned peaks including the ones at 2144,2093,2019 cm-‘. In our experiment we also expected to see the ketyl radical which is formed by reaction (7) or by recombination of H atoms and C,O in the matrix. Inoue and Suzuki [28] assigned from laser-induced fluorescence two absorptions at 1752 and 2334 cm-’ in the gas phase to HCCO. The peaks at 1787 and 1790 cm-’ in our matrix experiments are rather far apart from the 1762 cm-’ absorption. Such a large matrix shift would be unusual for a normal molecule. Ab initio calculations by Harding [29] gave a bent structure, the H-C-C angle is 132O and the O-C-O angle is 166O. It is not impossible

J. Dommen et al. / Vibrational spectrum ojethynol

that the influence of the matrix is larger on such an open shell molecule leading to such large frequency shifts. If the reaction was performed using setup A and sampling occurring through the pinhole, mainly CO and C,H, were produced. The identification of H&O, CH,, HC,H, H&CO was not unequivocal, because of very weak absorption lines. This result agrees with that of Faubel and Wagner [6]. In none of the experiments of both types an identification for ethynol could be found.

11

ketene rather than ethynol. (2) In the reaction H + C,O, no ethynol could be detected. The simultaneous condensation of C,O and H atoms is an inefficient source for the production of the ketyl radical. (3) The reaction of excited argon atoms with propiolic acid did not give any evidence for ethynol. The identification of HAr+ indicates a hydrogen abstraction mechanism rather than a decarbonylation.

5.4. Ar* + HCCCOOH Electron ionization of propiolic acid led to a decarbonylation producing H-OH+ [4]. This cation was then neutralized and again ionized to record the mass spectrum of ethynol and prove the existence of the neutral molecule. With the help of excited argon atoms Ar*, we hope to induce in propiolic acid the same type of decomposition. For this reaction we used the open-ended tube (setup B). In the infrared spectrum many new absorption lines appeared of which only a few could be assigned. Products identified were C,O,, C,O, ArH+, HCO, H&CO. The CO band was overlapped by propiolic acid. Since propiolic acid slowly decomposed even in the storage bulk yielding CO, and C,H,, their amount produced in the reaction could not be determined. The most prominent unassigned peak appeared at 2151, 2019, 1653,113l and 657 cm-‘. The peak at 2019 cm-’ also appeared in the reaction mixture of hydrogen atoms with C,O,. All unidentified absorption lines can be ruled out as belonging to ethynol. The appearance of HAr+ indicates a hydrogen abstraction rather than a decarbonylation as the main reaction route.

6. Conclusion The theoretical calculations show that for quasilinear molecules with multiple bonds the inclusion of electron correlation markedly affects the bending force constants. From the experiments the following conclusions can be drawn: (1) The reaction OH + CH,C=CH yields

Acknowledgement The authors wish to thank Professor A. Bauder for his interest in this work and helpful discussions. The generous grant of free computer time by the ETH-Zurich computation center and financial support by the Schweizerischer Nationalfonds (project No. 2.223-0.84) are gratefully acknowledged.

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