- Email: [email protected]
Laser magnetic resonance detection of rotational transitions in CH2
1 Octobrr
1979
Laser magnetic re~)rxmce spwtrx obsened at 163.0 Nrn hate been identified as pure rotntiontl transit;om. in the _gro~nd ekctronicst.rtc t3Br) of the meth)iene r.tdiLltI_The idr’ntdication was based on the observation of h>perfine spin triplets zmd by isotopic substirutions inxoIring!deutcriurn and r3C*.xs well as other chemicat 2nd spectroscopic er idsncc_ A IOR pressure ri2nlcproduced by reacrin~ discharged iiuorine with rnetiune \*a* iound to be an rxceiient new spectroscopic
sourceof gound-statcCII? rddicaIs_
The methylene biradicai, CH,, observed with electron panmagnetic resonance *%nd optical [21 spectroscopic techniques: continues TVbe an extremely important species for experimentalists and theoreticians * aIikt_ After several bears of uncertainty about its pound state, there is n01ir genera: agreement that it is o bent triplet stite (3BI)_ The avaiIabIerotational constants of CH, are c&t&ted from low-resolution opticai data [2] and EPR data [I 1 and are accurate to about 30% By comparison, the e_xtremeIysensitive technique of far-infixed Iaser magnetic resonance (IXR) spectroscopy [4j provides rotational constants and structural information with IO0 times greater ICCUracy- Because of the present controversy about the sin-
glet-triplet splitting ( 1At - ;B,) in methyiene [5---71, 3 direct determination of its groundstate geometry is very desirnble. Furthermore. an sccurate structurrtl determination would provide frequencies which could serve as the basis for astronomical searches for CH, in intenteiiar
gas ciouds_
This communication
reports the observation
al transitions in the ground electronic state of CH2_ Fig_ I shows an LhlR spectrum identified with CHZ_ Although the quantum states are not yet assigned. the large signal-to-noise ratio indicates the sensitivity of LMR for detecting CH2_ In this experiment_ the optic&y pumped far-infrared (FIR) Iaser spectrometer [S] was oscillating OR the 163.0 pm Iaser line of methanol, with its electric vector polarized
* NBS/NRC Postdoctoral Research Associate 1975--1977_ Present address Bell Laboratories. Mm-ray Hill. New Jersey 07974. USA_ ** See reF_ [I) for a concise review sod extensive bibliography
f
ofthisaxez For a
244
recent reviewseeref. [3]_
of sev-
eral LMR absorptions which are attributed to rotation-
perpendicular
to the
magnetic field (a poiarization)_ The methanol tnnsition was pumped with the 10 m R(38) line of a cw
CO, laser_ first-derivative presentation was effected using 1 kHz magnetic fieid modulation with an amplitude of about 0.0004 T (low4 T = I G) and phase-sen-
1 October
CIIC\IIC \L 1’111 SIC-S LETTtlRS
I-is- I. The O-O.2 T portton of the 163-O pm taser nugnetic In) pohriz.ation_The I or - si_rnzdrsignatc tr.msltions r~hichshift to higher or Io\irr firld respcctixely ~hrn the Iaser fiequcncr is incrmscd_
re~orxuicr’spcctrunl of CII~ (3B1) in perpcndm.hr
sitive detection_ In all. eleven sets of hypcrfine trlplcts wxe observed at this frequency in u poIarization; eight triplets are shown in fig. 1. centered tit 0.0472, 0.0510,0.l3S4,0.~6S7,0_172S, 0_1900,0_1922_and 0.1952 i_ Three additional trlplets wzre observed at 0_%29,0.663_0 and 0.7550 T_ In the other poIari/ation (zr)_ tbrcc tripkts
O-1361 0_0002 The length
wcrc obserwd
at O_OS5 I_
and 02288 T_ The uncertainty is about T_ observation of rotational spectra in this W;IWregion agrees with Bzmheim’s estimates f9] of the rotational enemy lcwls of C&l, based on optical [21 and EPR [ 1 JO--121 data_ Although these cstimates do not permit identification or assignment of the observed traxsitions, they do indicate this spectral region to be particuIarly dense in B-type rotation31 transitions. l-lyperfine triplets and singlets are both expected in CM, LMR spectra: howecer_ both are not likely to occur with the same laser line_ No attempt was made to analyse hyperfine singet spectra in these preliminary studies since they could well be due to other free radicals_ The lack of recognizable patterns in this observed spectrum precludes an assignment at present; however, the smali number of widely spaced lines indicate transitions involving low rotational quantum numbers. Both the ratio of the hyperfine splitting
1979
to observed linewidth and the direction of the magnetic f&d shrft LXith Idscr frequency vary for these lines, indicatmg that the observed spectrum may be due co rr.msitions imolving different spin or vibrational statesThe obsened hypcrtine triplets could be attributed to other radicals containing 311 I = 1 nucle~~s such as nitrogen Oi fwo I = l/3. nuclei such rls fIuorine_ Our m~.~surcments indicate fluortnecontaininS radicals c.m be disrcgrdcd since sources lacking fluorine also duplicated the spectrum, Ammonia: was substituted for methane in order to determine whether the tripIets \\ere 3 resldt of ;L nitrogen impurity. Well-known NH [ 131 and Nl I-, [ 141 spectra resulted: however. none of the features oi fig. 1 was reproduced. Similarly, mistures of oxygen and nitrogen passed through the- microwave dischxgc failed to gensratc the observed spectrum. The identification of CH, as the carrier responsible for the transitions sl~own in fig. 1 rests on the previously mentioned spectroscopx ecidence as well JS the following cl~crnical cvidrnccr
CI-2, was formed
by the rc-
action of fhwrine ~fonls with methane inside the laser cs\it> _ Helium and fluorine at pressures of 126 and 0.4 PA (752 mlorr = 1.0 Pa). respectively. were passed through a microwave discharge rend mised with 0.4 P_J of rncthanc in d nozzle !ocattd 30 mm from the axis of tbc laser. A blue flame rcsuited and its intensity maximized under approximately the same conditions ;ts the Ck!, signI_ CH spectra, also observed in this flame [ 157 _mrtsimized under very simdar conditions. A number of other sources. summsrired in table 1, Table 1 Et-fax of source chemistry and isoropic substitution upon the reIati\e LVR si~at intensities in the 163 ,um o spectrum bet\teen 0 snd O-2 T Source
--
Is0 topic
Signal intensity (arb_ units)
purity (7) obserwd “CH< + ICHtN2 + S C2H7_+0 CzHq + F CH3Tti‘=CD+ f F 13CH4 * F ‘2CD2H2 •t-F
a)
99 1 99 ;;
10055 20=5 2055 3025 LO%5 <2 752.
182 2
predicted 3 r 100 0 10
17
Prediction based on the abstraction mechanism: + CH2 + 2HF_
CH4 * 2F
245
Rcfercnces 1I j I<_1. Bernl!eim .md XII. Ch!cn. J_ Chcm_ l’h> L 66 ! 1977) 570x
f;l
1.1:. tf~rrison.
.\caounts Chsm. K;-\l_ l-renson.
Rex
7 (197-l)
378.
[ 4 1 P_B_1h ies and
in: Lecture notes *n physics. Vol. 43: Laser Spcctroscop_\ _ Procecdin~z of rhc 2nd Intrrn~tion~l Conference. cds. S. IIxorhc. J.C. I’cb.!? PcyrooI.!. T-N’. II;inszh and %I‘. f ixris (Springer. Ikrlin. 1975) p_ 131. [ 5 1 P-1 _ Zittsl. G_B_ Lllison, S.k_ OXed. I-_ I Ierbst. W-C_ Lineberax m!d \\‘.I’_ Rrinlurdt. J_ Am. Chw!. Sot_ 96 (1976) 37131_ 161 J. Danon. S.V. ITIsrth. D. I elmann, II. Zxh.!rias. <-II_ Du:gn and K_II_ WeIge. Chem. 1%~s. 29 ( 197s) 345: RX Lengzl and RX. &re. J. Am. Chem. Sot.. to be publisI!ed_ [7] R-R. Lucchesc and II.1 _ Schxfcr III. J. A111.Chem. Sot.
The e\idensc discussed
of the electronic
above indicates the presence ground state of CXI, in the lluorine-
methane t-k!nle_ A detaiIed sptctroscopk fitting and analysis of the spectra obtained thus far has not been possible; however the observation of further spectra with other laser fines should provide sufficient data for an accurate fit and a determination of accurate rotational constants of CH2_ When the LAIR spectra of CHI is assigned it wilt be possible to deduce an accurate molecular structure for the radical- it is afso possible that knowIedge of the spin-orbit interaction. coup!inp the 3B, and *A, states and the HCH bending frequency in the ‘B, state will. be obtained_ This will be
an important
contribution
of the CH3 photoelectron
toward the interpretation spectra
[5] _ LMR detection
has been &ed to study the reaction kinetics of HO-, [ 191 and HO [20] in flow reactors and similar studies of the chemistry of CH-, 3B, may also be possible. FinalIy, and perhaps m&t importantly, the discovery of this new source of triplet CH2 should provide other experiments with a valuabIe means to perform additional new sttidies on this radical_ 246
99 (1977) 6765. B. Roos and P.11. Siegbahn. J. Am. Chem_ Sot_ 99 ( 19771 7716; L.B. Harding and \\_A_ Goddard III. J_ Chem_ Phys- 67 f 1977) 1777: C-W_ Bauschlicher Jr. snd I_ Shatiitt. J. Am. Chem. Sot. 100 (1978) 739; S.-K_ Shih. S-D_ Pe\erimhotT. R J_ Buenher and \I_ Peri;. Chenr Ph> s. Letters 55 (1978) 206: L-B_ Harding and W-A_ Goddard 111.Chem_ Phyr Letters 5.5 (197s) 117_ [S] KM_ Esenson. D.A. Jennings, 1-R. Petersen. J-4_ Mucha. JJ_ Jimenez, RX. Charlton and C-l. floward. IEEL 1. Quantum Flectron_ QE-13 (1977) 442_ [ 91 R-A. Bernhrim. prhate communication. [IO] E_ Wasserman. R-S_ Hutton, VJ. Ku& and WA. Yaser. J. Chem. Phyr 55 (1971) 2593. [I I j R.A. Bernheim, T. AdI, H.W. Bernard. A. Songco. P_S_Wmg. R_ Wang. LS. Wood and PS. SkeU. J_ Chem. Phyr 64 (1976) 2747. [I?] B-R_ BickneII. W_RJl_ Graham and W_ Weltner Jr_. J_ Chem. Phys- 64 (1976) 3319_ [ 131 H.E. Radford and bfbl. Litvak. Chem. Phys. Letters 34 (1975) 561; F-D_ Wayne and H-E. Radford. Mol. Phys. 32 (1976) 1407_ [ 141 P-B_ Davies, D-K. Russell. B.A. Thrush and F-D_ Wayne, J. Chem- Phys. 62 (1975) 3739: P-B_ Davies, D-K. Russell. BA_ Thrush and H-E_ Radford, Chem_ Phys. Letters 42 (X976) 35.
ClI1-\lIC\L
[ 151 J-l-. Ilou~cn.
PHI strs
J \. \l~rchj. I)./\. Jwnrn;~ and EM. I‘\enson. J. \~ol. Sprctry. 72 t 197s) 463. [ 161 J-R. f¬-shy. D. Lurzis, I‘_ Pru>s and I)_ <;urman. J. Phy\-_ Chem. 78 (197-I) 311; I-TX. Jones snd K.D. Bs)es. 14rh S) mposium Inttrnationat on Combustion (Combustion Instrtutr. Pittsbur~. 1973) p_ 27?_
LI‘T-rt-RS
1 Gctobes
1979
[ 171 RJ. S+ k.di) and 6-U. l..enson_ paper I‘U-S_Thirty-Third S) mposium on Volsrular Specrrorcopl , The Ohio State L!nirasil) .Columbu~.Gh~o. June 11-16. 1978. [ ISI J-t\. Vkbrt. DA_ Jenr.in:_s. K.V. ELenson and J.T. iIou:en. J. Vol. Spcctry. 68 (1977) 112. 1191 C.J. IIowJrd ,md K.11. Ll~cnson, Gcopbys. Res. Letters 10 (1977) 537. (Xl] C J. fIox\,xd and K.U. i:\enson, J. Chcm. Phys. 61 (1974) 1943.
247
1979
Laser magnetic re~)rxmce spwtrx obsened at 163.0 Nrn hate been identified as pure rotntiontl transit;om. in the _gro~nd ekctronicst.rtc t3Br) of the meth)iene r.tdiLltI_The idr’ntdication was based on the observation of h>perfine spin triplets zmd by isotopic substirutions inxoIring!deutcriurn and r3C*.xs well as other chemicat 2nd spectroscopic er idsncc_ A IOR pressure ri2nlcproduced by reacrin~ discharged iiuorine with rnetiune \*a* iound to be an rxceiient new spectroscopic
sourceof gound-statcCII? rddicaIs_
The methylene biradicai, CH,, observed with electron panmagnetic resonance *%nd optical [21 spectroscopic techniques: continues TVbe an extremely important species for experimentalists and theoreticians * aIikt_ After several bears of uncertainty about its pound state, there is n01ir genera: agreement that it is o bent triplet stite (3BI)_ The avaiIabIerotational constants of CH, are c&t&ted from low-resolution opticai data [2] and EPR data [I 1 and are accurate to about 30% By comparison, the e_xtremeIysensitive technique of far-infixed Iaser magnetic resonance (IXR) spectroscopy [4j provides rotational constants and structural information with IO0 times greater ICCUracy- Because of the present controversy about the sin-
glet-triplet splitting ( 1At - ;B,) in methyiene [5---71, 3 direct determination of its groundstate geometry is very desirnble. Furthermore. an sccurate structurrtl determination would provide frequencies which could serve as the basis for astronomical searches for CH, in intenteiiar
gas ciouds_
This communication
reports the observation
al transitions in the ground electronic state of CH2_ Fig_ I shows an LhlR spectrum identified with CHZ_ Although the quantum states are not yet assigned. the large signal-to-noise ratio indicates the sensitivity of LMR for detecting CH2_ In this experiment_ the optic&y pumped far-infrared (FIR) Iaser spectrometer [S] was oscillating OR the 163.0 pm Iaser line of methanol, with its electric vector polarized
* NBS/NRC Postdoctoral Research Associate 1975--1977_ Present address Bell Laboratories. Mm-ray Hill. New Jersey 07974. USA_ ** See reF_ [I) for a concise review sod extensive bibliography
f
ofthisaxez For a
244
recent reviewseeref. [3]_
of sev-
eral LMR absorptions which are attributed to rotation-
perpendicular
to the
magnetic field (a poiarization)_ The methanol tnnsition was pumped with the 10 m R(38) line of a cw
CO, laser_ first-derivative presentation was effected using 1 kHz magnetic fieid modulation with an amplitude of about 0.0004 T (low4 T = I G) and phase-sen-
1 October
CIIC\IIC \L 1’111 SIC-S LETTtlRS
I-is- I. The O-O.2 T portton of the 163-O pm taser nugnetic In) pohriz.ation_The I or - si_rnzdrsignatc tr.msltions r~hichshift to higher or Io\irr firld respcctixely ~hrn the Iaser fiequcncr is incrmscd_
re~orxuicr’spcctrunl of CII~ (3B1) in perpcndm.hr
sitive detection_ In all. eleven sets of hypcrfine trlplcts wxe observed at this frequency in u poIarization; eight triplets are shown in fig. 1. centered tit 0.0472, 0.0510,0.l3S4,0.~6S7,0_172S, 0_1900,0_1922_and 0.1952 i_ Three additional trlplets wzre observed at 0_%29,0.663_0 and 0.7550 T_ In the other poIari/ation (zr)_ tbrcc tripkts
O-1361 0_0002 The length
wcrc obserwd
at O_OS5 I_
and 02288 T_ The uncertainty is about T_ observation of rotational spectra in this W;IWregion agrees with Bzmheim’s estimates f9] of the rotational enemy lcwls of C&l, based on optical [21 and EPR [ 1 JO--121 data_ Although these cstimates do not permit identification or assignment of the observed traxsitions, they do indicate this spectral region to be particuIarly dense in B-type rotation31 transitions. l-lyperfine triplets and singlets are both expected in CM, LMR spectra: howecer_ both are not likely to occur with the same laser line_ No attempt was made to analyse hyperfine singet spectra in these preliminary studies since they could well be due to other free radicals_ The lack of recognizable patterns in this observed spectrum precludes an assignment at present; however, the smali number of widely spaced lines indicate transitions involving low rotational quantum numbers. Both the ratio of the hyperfine splitting
1979
to observed linewidth and the direction of the magnetic f&d shrft LXith Idscr frequency vary for these lines, indicatmg that the observed spectrum may be due co rr.msitions imolving different spin or vibrational statesThe obsened hypcrtine triplets could be attributed to other radicals containing 311 I = 1 nucle~~s such as nitrogen Oi fwo I = l/3. nuclei such rls fIuorine_ Our m~.~surcments indicate fluortnecontaininS radicals c.m be disrcgrdcd since sources lacking fluorine also duplicated the spectrum, Ammonia: was substituted for methane in order to determine whether the tripIets \\ere 3 resldt of ;L nitrogen impurity. Well-known NH [ 131 and Nl I-, [ 141 spectra resulted: however. none of the features oi fig. 1 was reproduced. Similarly, mistures of oxygen and nitrogen passed through the- microwave dischxgc failed to gensratc the observed spectrum. The identification of CH, as the carrier responsible for the transitions sl~own in fig. 1 rests on the previously mentioned spectroscopx ecidence as well JS the following cl~crnical cvidrnccr
CI-2, was formed
by the rc-
action of fhwrine ~fonls with methane inside the laser cs\it> _ Helium and fluorine at pressures of 126 and 0.4 PA (752 mlorr = 1.0 Pa). respectively. were passed through a microwave discharge rend mised with 0.4 P_J of rncthanc in d nozzle !ocattd 30 mm from the axis of tbc laser. A blue flame rcsuited and its intensity maximized under approximately the same conditions ;ts the Ck!, signI_ CH spectra, also observed in this flame [ 157 _mrtsimized under very simdar conditions. A number of other sources. summsrired in table 1, Table 1 Et-fax of source chemistry and isoropic substitution upon the reIati\e LVR si~at intensities in the 163 ,um o spectrum bet\teen 0 snd O-2 T Source
--
Is0 topic
Signal intensity (arb_ units)
purity (7) obserwd “CH< + ICHtN2 + S C2H7_+0 CzHq + F CH3Tti‘=CD+ f F 13CH4 * F ‘2CD2H2 •t-F
a)
99 1 99 ;;
10055 20=5 2055 3025 LO%5 <2 752.
182 2
predicted 3 r 100 0 10
17
Prediction based on the abstraction mechanism: + CH2 + 2HF_
CH4 * 2F
245
Rcfercnces 1I j I<_1. Bernl!eim .md XII. Ch!cn. J_ Chcm_ l’h> L 66 ! 1977) 570x
f;l
1.1:. tf~rrison.
.\caounts Chsm. K;-\l_ l-renson.
Rex
7 (197-l)
378.
[ 4 1 P_B_1h ies and
in: Lecture notes *n physics. Vol. 43: Laser Spcctroscop_\ _ Procecdin~z of rhc 2nd Intrrn~tion~l Conference. cds. S. IIxorhc. J.C. I’cb.!? PcyrooI.!. T-N’. II;inszh and %I‘. f ixris (Springer. Ikrlin. 1975) p_ 131. [ 5 1 P-1 _ Zittsl. G_B_ Lllison, S.k_ OXed. I-_ I Ierbst. W-C_ Lineberax m!d \\‘.I’_ Rrinlurdt. J_ Am. Chw!. Sot_ 96 (1976) 37131_ 161 J. Danon. S.V. ITIsrth. D. I elmann, II. Zxh.!rias. <-II_ Du:gn and K_II_ WeIge. Chem. 1%~s. 29 ( 197s) 345: RX Lengzl and RX. &re. J. Am. Chem. Sot.. to be publisI!ed_ [7] R-R. Lucchesc and II.1 _ Schxfcr III. J. A111.Chem. Sot.
The e\idensc discussed
of the electronic
above indicates the presence ground state of CXI, in the lluorine-
methane t-k!nle_ A detaiIed sptctroscopk fitting and analysis of the spectra obtained thus far has not been possible; however the observation of further spectra with other laser fines should provide sufficient data for an accurate fit and a determination of accurate rotational constants of CH2_ When the LAIR spectra of CHI is assigned it wilt be possible to deduce an accurate molecular structure for the radical- it is afso possible that knowIedge of the spin-orbit interaction. coup!inp the 3B, and *A, states and the HCH bending frequency in the ‘B, state will. be obtained_ This will be
an important
contribution
of the CH3 photoelectron
toward the interpretation spectra
[5] _ LMR detection
has been &ed to study the reaction kinetics of HO-, [ 191 and HO [20] in flow reactors and similar studies of the chemistry of CH-, 3B, may also be possible. FinalIy, and perhaps m&t importantly, the discovery of this new source of triplet CH2 should provide other experiments with a valuabIe means to perform additional new sttidies on this radical_ 246
99 (1977) 6765. B. Roos and P.11. Siegbahn. J. Am. Chem_ Sot_ 99 ( 19771 7716; L.B. Harding and \\_A_ Goddard III. J_ Chem_ Phys- 67 f 1977) 1777: C-W_ Bauschlicher Jr. snd I_ Shatiitt. J. Am. Chem. Sot. 100 (1978) 739; S.-K_ Shih. S-D_ Pe\erimhotT. R J_ Buenher and \I_ Peri;. Chenr Ph> s. Letters 55 (1978) 206: L-B_ Harding and W-A_ Goddard 111.Chem_ Phyr Letters 5.5 (197s) 117_ [S] KM_ Esenson. D.A. Jennings, 1-R. Petersen. J-4_ Mucha. JJ_ Jimenez, RX. Charlton and C-l. floward. IEEL 1. Quantum Flectron_ QE-13 (1977) 442_ [ 91 R-A. Bernhrim. prhate communication. [IO] E_ Wasserman. R-S_ Hutton, VJ. Ku& and WA. Yaser. J. Chem. Phyr 55 (1971) 2593. [I I j R.A. Bernheim, T. AdI, H.W. Bernard. A. Songco. P_S_Wmg. R_ Wang. LS. Wood and PS. SkeU. J_ Chem. Phyr 64 (1976) 2747. [I?] B-R_ BickneII. W_RJl_ Graham and W_ Weltner Jr_. J_ Chem. Phys- 64 (1976) 3319_ [ 131 H.E. Radford and bfbl. Litvak. Chem. Phys. Letters 34 (1975) 561; F-D_ Wayne and H-E. Radford. Mol. Phys. 32 (1976) 1407_ [ 141 P-B_ Davies, D-K. Russell. B.A. Thrush and F-D_ Wayne, J. Chem- Phys. 62 (1975) 3739: P-B_ Davies, D-K. Russell. BA_ Thrush and H-E_ Radford, Chem_ Phys. Letters 42 (X976) 35.
ClI1-\lIC\L
[ 151 J-l-. Ilou~cn.
PHI strs
J \. \l~rchj. I)./\. Jwnrn;~ and EM. I‘\enson. J. \~ol. Sprctry. 72 t 197s) 463. [ 161 J-R. f¬-shy. D. Lurzis, I‘_ Pru>s and I)_ <;urman. J. Phy\-_ Chem. 78 (197-I) 311; I-TX. Jones snd K.D. Bs)es. 14rh S) mposium Inttrnationat on Combustion (Combustion Instrtutr. Pittsbur~. 1973) p_ 27?_
LI‘T-rt-RS
1 Gctobes
1979
[ 171 RJ. S+ k.di) and 6-U. l..enson_ paper I‘U-S_Thirty-Third S) mposium on Volsrular Specrrorcopl , The Ohio State L!nirasil) .Columbu~.Gh~o. June 11-16. 1978. [ ISI J-t\. Vkbrt. DA_ Jenr.in:_s. K.V. ELenson and J.T. iIou:en. J. Vol. Spcctry. 68 (1977) 112. 1191 C.J. IIowJrd ,md K.11. Ll~cnson, Gcopbys. Res. Letters 10 (1977) 537. (Xl] C J. fIox\,xd and K.U. i:\enson, J. Chcm. Phys. 61 (1974) 1943.
247