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Searching for HNC
Volume 34, ntimber 2
CHEMICALPHYS!CSLETTERS
15 July 1975
SEARCHING FOR HNC G.L. BLACKMAN, Department
R-D. BROWN,P.D. GODFREY and H-1. GUNN
of Chemistry.
Morrash
University.
Oayton,
Victoria
3168.
Australia
Recetied 21 January 197.5 Revised manuscript received 20 hIarch 1975
A search for the microwave spectrum of HNC molecules present in a sample of HCN is reported. A lower limit to the zero point energy difference between the two molecules of 0.47kO.02 eV is deduced.
Much interest has been focused on the species HNC (hydrogen isocyanide) since Snyder and Buhl [l] dis-
of HNC in HCN at 300 R to be i= 3X 10-ll.
covered a strong emission line at 90665 (5 1) MHz in the radio sources W5 1 and DR2 1. This was subsequently ascribed to the J = 1 + 0 transition of HNC. In this
Although this concentration is below the limits normally detectable with a conventional microwave spectrometer it was considered desirable for a first experiment to es‘Lablishan experimenrd upper limit for the concentra-
letter we report an attempt to observe the J = 1 -+ 0 microwave absorption of HNC assuming that it is in equilibrium with HCN (hydrogen cyanide) under the
tion of HNC in HCN and hence establish a Iower limit for the energy of isomerisation. The spectrometer consisted of a standard X-band
conditions of our experiment. So far HNC has been observed in the laboratory
waveguide cell with 30 kHz Stark modulation, a phase locked OK1 90VlOA Klystron source at 90.6 GHz and a Baytron IE-3 1 pointcontactdiode detector followed
only in frozen matrices of Ar and CO, [2,3], ing photolysis of CH3N3 or HCN. Vibrational
followfrequen-
cies ul, u2, u3 were measured and force constants
cal-
culated. The molecuiar geometry may be estimated From these by mears of Badger’s ru:es and thus the rotational transition frequency may be roughly predicted. Such evidence supports the proposition that the line of Snyder and 9u!ll may be attributed to HNC. Furthermore ab initio calculations of the HCN-HNC isomerisation have been carried out by Booth and
Murrell [4] and more elaborately by Pearson et al. [5]. The latter group also concludes that the 90665 MHz line is due to HNC. In particular the J = 1 + 0 transition was shown to be about 90.5 GHz (50.2%). These calculations indicated that HNC is less stable thm HCN by 0.63 eV per molecule* _Thus on this basis one would expect an equilibrium mole fraction * Boolh and MurielI obtain an energy difference of 0.37 eV molecule+, corresponding to a mole fraction of 8 X10-’ at 300 K.
by a 30 kHz low noise pre-amplilrier and phase sensitive detection system. In order to further improve sen-
sitivity, 2 multichannel analyser signal-averaging system was available to add successive scans over the frequency range of interest.
The sensitivity
was calibrated, using
of the spectrometer
single scan only, by observation of the vibrational satellites of the J = 1 + 0 transition 2
of HCN. In this way the J= 1 + 0 transition of the 02OO state of H12C14N at 89 088 MHz was observed
under the conditions of maximum spectrometer sensitivity: 1OPa pressure and 500 ~‘a of detected power. At 300 K this vibrationa! satellite has a calculated intensity of 1 X10m3 of the main line of HL2C14N and could be observed under the conditions appropriate to the HNC experiment with a signal to noise ratio of 5X103. It was verified that the sensitivity did not vary
markedly with frequency by observing the same vibrational satellite in H13C14N at 86748 MHz where a signal to noise ratio of = 50 was obtained_ From these that 2 mole fraction 2X10--’
figures it was estimated
241
Volume 34, nlin-lber ‘2
15 July 1975
CHEMCAL PfIySICS LETTERS
o-.
50680
90665
90650
MICROWAVE
FREQUENCY
( MHz)
Fig. 1. Experimental microwav: absorption curves for 206 scms of 300 s duration. The 51 MHz uncertainty in the rest frequency of the ob.Qrved interstellar line is shown by (a) whilst the 4 MHz fwhm estimated for the I= 1 + 0 line of HNC and the conditions used in the experiment
is denoted
by (b). The gs pressure in the cell is lOPa.
could be observed in a single scan with a unity signal to noise ratio. Fig. 1 shows the results of the HNC search after 206 scans of 300 s d ration over a 32 MHz bandwidth. The rest frequency of the interstellar line observed by Snyder and BUN [ 11, togefher with their error limits is also shown. From the pressure used in the cell and the tentative hyperfine structure of HNC as suggested by Pearson et al. [5] the line is expected to be = 4% MHz wide (fwhm) and to be well approximated by a single lorentzian. There is no strong evidence for a line of this shape at 90 66.5 MHz. Furthermore application of the Fisher F-test has failed to show.any evidence of the HNC line in this data. From the spectrameter sensitivity and the enhancement factor cm) for tho signal-averaged data, we may c&clude that the fm:ll sensitivity reached would have been sufficient to detect a mole fraction of x 1 .4 X 1CJL8 (unkertdn to within a factor of 2) and
that this is the upper limit for HNC in HCN, provided that the J = 1 -+ 0 line of HNC is, as assumed, at 90 665 MHz. This conclusion furthermore rests on the assumption that the expected line strengths for the HNC and HCN transitions are the same. This would require that their respective dipole moments should be equal and that there is no loss of intensity due to spread over the 14N nuclear quadrupole hyperfine
components for each species. SCF calculations [5] indicate that the HCN dipole moment is slightly larger than that of HNC (3.i2 debye compared to 2.91 debye) and that the quadrupole coupling for HNC (eQq = 0.93 MHz) is smaller than for HCN (eQq z -3.65 MHz). However the effect of modifying the derived detectable mole fraction to account foi these differences favours HCN by only -_”7% over HNC. This difference is smaller than the experimental uncertainties in the measurement and is not significant. The upper limit to’& HNC/HCN concentrzztion ratio at 300 K of l.., X IO-8 sets a lower Limit to the zero-
Volume 34, number 2
CHEMICAL PHYSICSLEITERS
enerm difference between these molecules of b.47+0.02eV (45 kJ mol-I). It seems likely that the HCN/HNC equilibrium is rapidly obtained at 300 K except at very low pressures (the theoretical studies 14,5] imply exceedingly slow unimolecular rearrangement). We are continuing attempts to obtain direct laboratory identification of the 90 665 MHz line. point
This work was supported by the Australian Research Grants Committee. We thank Drs. M.F. O’Dwyer and J,E, Kent for the loan of the muhichannel an2lyser used for signal averaging.
15 ruiy 1975
References r11 L. Snyder and D. Buhl, Bull. Am. Astron. Sot. 3 (1971) 388. 121 D.E. Mulligan and ME. J~COX,J. Chem. Phys. 37 (1963) 1687.
I31 D.E. hWigi?n and ME. Jacox, J. Chem. Phys. 47 (1967) 278.
[41 D. Booth and J.N. hiwell. blol. Phys. 24 (1972) 1117. 151 P.K. Peuson, C.L. Bhckman, HF. Schaefer. B. Roos and U. Wahigren, Astrophys. J. 184 (1973) L19.
CHEMICALPHYS!CSLETTERS
15 July 1975
SEARCHING FOR HNC G.L. BLACKMAN, Department
R-D. BROWN,P.D. GODFREY and H-1. GUNN
of Chemistry.
Morrash
University.
Oayton,
Victoria
3168.
Australia
Recetied 21 January 197.5 Revised manuscript received 20 hIarch 1975
A search for the microwave spectrum of HNC molecules present in a sample of HCN is reported. A lower limit to the zero point energy difference between the two molecules of 0.47kO.02 eV is deduced.
Much interest has been focused on the species HNC (hydrogen isocyanide) since Snyder and Buhl [l] dis-
of HNC in HCN at 300 R to be i= 3X 10-ll.
covered a strong emission line at 90665 (5 1) MHz in the radio sources W5 1 and DR2 1. This was subsequently ascribed to the J = 1 + 0 transition of HNC. In this
Although this concentration is below the limits normally detectable with a conventional microwave spectrometer it was considered desirable for a first experiment to es‘Lablishan experimenrd upper limit for the concentra-
letter we report an attempt to observe the J = 1 -+ 0 microwave absorption of HNC assuming that it is in equilibrium with HCN (hydrogen cyanide) under the
tion of HNC in HCN and hence establish a Iower limit for the energy of isomerisation. The spectrometer consisted of a standard X-band
conditions of our experiment. So far HNC has been observed in the laboratory
waveguide cell with 30 kHz Stark modulation, a phase locked OK1 90VlOA Klystron source at 90.6 GHz and a Baytron IE-3 1 pointcontactdiode detector followed
only in frozen matrices of Ar and CO, [2,3], ing photolysis of CH3N3 or HCN. Vibrational
followfrequen-
cies ul, u2, u3 were measured and force constants
cal-
culated. The molecuiar geometry may be estimated From these by mears of Badger’s ru:es and thus the rotational transition frequency may be roughly predicted. Such evidence supports the proposition that the line of Snyder and 9u!ll may be attributed to HNC. Furthermore ab initio calculations of the HCN-HNC isomerisation have been carried out by Booth and
Murrell [4] and more elaborately by Pearson et al. [5]. The latter group also concludes that the 90665 MHz line is due to HNC. In particular the J = 1 + 0 transition was shown to be about 90.5 GHz (50.2%). These calculations indicated that HNC is less stable thm HCN by 0.63 eV per molecule* _Thus on this basis one would expect an equilibrium mole fraction * Boolh and MurielI obtain an energy difference of 0.37 eV molecule+, corresponding to a mole fraction of 8 X10-’ at 300 K.
by a 30 kHz low noise pre-amplilrier and phase sensitive detection system. In order to further improve sen-
sitivity, 2 multichannel analyser signal-averaging system was available to add successive scans over the frequency range of interest.
The sensitivity
was calibrated, using
of the spectrometer
single scan only, by observation of the vibrational satellites of the J = 1 + 0 transition 2
of HCN. In this way the J= 1 + 0 transition of the 02OO state of H12C14N at 89 088 MHz was observed
under the conditions of maximum spectrometer sensitivity: 1OPa pressure and 500 ~‘a of detected power. At 300 K this vibrationa! satellite has a calculated intensity of 1 X10m3 of the main line of HL2C14N and could be observed under the conditions appropriate to the HNC experiment with a signal to noise ratio of 5X103. It was verified that the sensitivity did not vary
markedly with frequency by observing the same vibrational satellite in H13C14N at 86748 MHz where a signal to noise ratio of = 50 was obtained_ From these that 2 mole fraction 2X10--’
figures it was estimated
241
Volume 34, nlin-lber ‘2
15 July 1975
CHEMCAL PfIySICS LETTERS
o-.
50680
90665
90650
MICROWAVE
FREQUENCY
( MHz)
Fig. 1. Experimental microwav: absorption curves for 206 scms of 300 s duration. The 51 MHz uncertainty in the rest frequency of the ob.Qrved interstellar line is shown by (a) whilst the 4 MHz fwhm estimated for the I= 1 + 0 line of HNC and the conditions used in the experiment
is denoted
by (b). The gs pressure in the cell is lOPa.
could be observed in a single scan with a unity signal to noise ratio. Fig. 1 shows the results of the HNC search after 206 scans of 300 s d ration over a 32 MHz bandwidth. The rest frequency of the interstellar line observed by Snyder and BUN [ 11, togefher with their error limits is also shown. From the pressure used in the cell and the tentative hyperfine structure of HNC as suggested by Pearson et al. [5] the line is expected to be = 4% MHz wide (fwhm) and to be well approximated by a single lorentzian. There is no strong evidence for a line of this shape at 90 66.5 MHz. Furthermore application of the Fisher F-test has failed to show.any evidence of the HNC line in this data. From the spectrameter sensitivity and the enhancement factor cm) for tho signal-averaged data, we may c&clude that the fm:ll sensitivity reached would have been sufficient to detect a mole fraction of x 1 .4 X 1CJL8 (unkertdn to within a factor of 2) and
that this is the upper limit for HNC in HCN, provided that the J = 1 -+ 0 line of HNC is, as assumed, at 90 665 MHz. This conclusion furthermore rests on the assumption that the expected line strengths for the HNC and HCN transitions are the same. This would require that their respective dipole moments should be equal and that there is no loss of intensity due to spread over the 14N nuclear quadrupole hyperfine
components for each species. SCF calculations [5] indicate that the HCN dipole moment is slightly larger than that of HNC (3.i2 debye compared to 2.91 debye) and that the quadrupole coupling for HNC (eQq = 0.93 MHz) is smaller than for HCN (eQq z -3.65 MHz). However the effect of modifying the derived detectable mole fraction to account foi these differences favours HCN by only -_”7% over HNC. This difference is smaller than the experimental uncertainties in the measurement and is not significant. The upper limit to’& HNC/HCN concentrzztion ratio at 300 K of l.., X IO-8 sets a lower Limit to the zero-
Volume 34, number 2
CHEMICAL PHYSICSLEITERS
enerm difference between these molecules of b.47+0.02eV (45 kJ mol-I). It seems likely that the HCN/HNC equilibrium is rapidly obtained at 300 K except at very low pressures (the theoretical studies 14,5] imply exceedingly slow unimolecular rearrangement). We are continuing attempts to obtain direct laboratory identification of the 90 665 MHz line. point
This work was supported by the Australian Research Grants Committee. We thank Drs. M.F. O’Dwyer and J,E, Kent for the loan of the muhichannel an2lyser used for signal averaging.
15 ruiy 1975
References r11 L. Snyder and D. Buhl, Bull. Am. Astron. Sot. 3 (1971) 388. 121 D.E. Mulligan and ME. J~COX,J. Chem. Phys. 37 (1963) 1687.
I31 D.E. hWigi?n and ME. Jacox, J. Chem. Phys. 47 (1967) 278.
[41 D. Booth and J.N. hiwell. blol. Phys. 24 (1972) 1117. 151 P.K. Peuson, C.L. Bhckman, HF. Schaefer. B. Roos and U. Wahigren, Astrophys. J. 184 (1973) L19.