The photoionization spectrum of the ionization threshold region of methylene has been recorded for the first time. The CH2 radical was produced in situ by successive hydrogen abstractions from methane precursor. The observed steplike onset corresponds to the vibrationless transition CH2+X̃ 2A1CH2X̃ 3B1 and leads to the adiabatic ionization energy of CH2 of 10.393±0.011 eV. This value is slightly higher than the nominal midrise of the threshold step structure, which is depressed by rotational autoionization effects. In a separate set of experiments, the threshold region of the CH2+ fragment from CH3 was recorded at room temperature. The fragment appearance energy was accurately determined by fitting to be 15.120±0.006 eV at 0 K. The combination of these two measurements provides the best current experimental value for the bond dissociation energy of the methyl radical, D0(H–CH2)=4.727±0.012 eV=109.0±0.3 kcal/mol (corresponding to 110.4±0.3 kcal/mol at 298 K), and yields ΔHf0(CH2,X̃ 3B1)=93.2±0.3 kcal/mol (93.3±0.3 kcal/mol at 298 K) and ΔHf0(CH2,ã  1A2)=102.2±0.3 kcal/mol (102.3±0.3 kcal/mol at 298 K). The latter makes the reaction CH2 (ã  1A2)+H2O→CH3+OH essentially thermoneutral, ΔHr0=0.0±0.3 kcal/mol.

1.
A.
Niehaus
,
Z. Naturforsch.
22A
,
690
(
1967
).
2.
W.
Reineke
and
K.
Strein
,
Ber. Bunsenges. Phys. Chem.
80
,
343
(
1976
).
3.
G.
Herzberg
,
Proc. R. Soc. London, Ser. A
262
,
291
(
1961
);
see also
G.
Herzberg
and
J.
Shoosmith
,
Nature (London)
183
,
1801
(
1959
).
4.
G.
Herzberg
,
Can. J. Phys.
39
,
1511
(
1961
).
5.
G.
Herzberg
and
J. W. C.
Johns
,
J. Chem. Phys.
54
,
2276
(
1971
).
6.
(a)
D. L.
Yeager
,
J. Chem. Phys.
105
,
8170
(
1996
);
(b)
J. A.
Nichols
,
D.
Heryadi
,
D. L.
Yeager
, and
J. T.
Golab
,
J. Chem. Phys.
100
,
2947
(
1994
);
(c)
N.
Russo
,
E.
Sicilia
, and
M.
Toscano
,
J. Chem. Phys.
97
,
5031
(
1992
);
(d)
D.
Gervy
and
G.
Verhaegen
,
Int. J. Quantum Chem.
12
,
115
(
1977
).
7.
D. D.
Wagman
et al., The NBS Tables of Chemical Thermodynamic Properties, NBS Tech. Note No. 270 [
J. Phys. Chem. Ref. Data
11
,
Suppl
.
2
(
1982
)].
8.
M. W.
Chase
et al., JANAF Thermochemical Tables, 3rd ed. [
J. Phys. Chem. Ref. Data
14
,
Suppl
.
1
(
1985
)].
9.
(a) L. V. Gurvich, I. V. Veyts, and C. B. Alcock, Thermodynamic Properties of Individual Substances (Hemisphere, New York, 1989), Vol. 1, Parts 1 and 2;
(b) L. V. Gurvich, I. V. Veyts, and C. B. Alcock, Thermodynamic Properties of Individual Substances (Hemisphere, New York, 1991), Vol. 2, Parts 1 and 2.
10.
J.
Berkowitz
,
G. B.
Ellison
, and
D.
Gutman
,
J. Phys. Chem.
98
,
2744
(
1994
).
11.
W. A.
Chupka
and
C.
Lifshitz
,
J. Chem. Phys.
48
,
1109
(
1967
).
12.
V. H.
Dibeler
,
M.
Krauss
,
R. M.
Reese
, and
F.
Harllee
,
J. Chem. Phys.
42
,
3791
(
1965
).
13.
W. A.
Chupka
,
J. Chem. Phys.
48
,
2337
(
1968
).
14.
K. E.
McCulloh
and
V. H.
Dibeler
,
J. Chem. Phys.
64
,
4445
(
1976
).
15.
M. Litorja, R. Asher, and B. Ruscic (unpublished).
16.
R. L.
Nuttall
,
A. H.
Laufer
, and
M. V.
Kilday
,
J. Chem. Thermodyn.
3
,
167
(
1971
).
17.
R. K.
Lengel
and
R. N.
Zare
,
J. Am. Chem. Soc.
100
,
7495
(
1978
).
18.
D.
Feldmann
,
K.
Meier
,
H.
Zacharias
, and
K. H.
Welge
,
Chem. Phys. Lett.
59
,
171
(
1978
).
19.
(a)
P. R.
Bunker
,
P.
Jensen
,
W. P.
Kraemer
, and
R.
Beardsworth
,
J. Chem. Phys.
85
,
3724
(
1986
);
(b)
P.
Jensen
and
P. R.
Bunker
,
J. Chem. Phys.
89
,
1327
(
1988
);
(c)
D. G.
Leopold
,
K. K.
Murray
,
A. E. Stevens
Miller
, and
W. C.
Lineberger
,
J. Chem. Phys.
83
,
4849
(
1985
).
20.
C. C.
Hayden
,
D. M.
Neumark
,
K.
Shobatake
,
R. K.
Sparks
, and
Y. T.
Lee
,
J. Chem. Phys.
76
,
3607
(
1982
).
21.
L. A.
Curtiss
,
K.
Raghavachari
,
G. W.
Trucks
, and
J. A.
Pople
,
J. Chem. Phys.
94
,
7221
(
1991
).
22.
R. S.
Grev
and
H. F.
Schaefer
III
,
J. Chem. Phys.
97
,
8389
(
1992
).
23.
K. A.
Peterson
and
T. H.
Dunning
, Jr.
,
J. Chem. Phys.
106
,
4119
(
1997
).
24.
N. L.
Doltsinis
and
P. J.
Knowles
,
J. Chem. Soc., Faraday Trans.
93
,
2025
(
1997
).
25.
M.
Litorja
and
B.
Ruscic
,
J. Chem. Phys.
107
,
9852
(
1997
).
26.
(a)
B.
Ruscic
and
J.
Berkowitz
,
J. Phys. Chem.
97
,
11451
(
1993
);
(b)
B.
Ruscic
and
J.
Berkowitz
,
J. Chem. Phys.
100
,
4498
(
1994
);
(c)
B.
Ruscic
and
J.
Berkowitz
,
J. Chem. Phys.
101
,
7795
(
1994
);
(d)
B.
Ruscic
and
J.
Berkowitz
,
J. Chem. Phys.
101
,
7975
(
1994
);
(e)
B.
Ruscic
and
J.
Berkowitz
,
J. Chem. Phys.
101
,
10936
(
1994
);
(f)
R. L.
Asher
and
B.
Ruscic
,
J. Chem. Phys.
106
,
210
(
1997
);
(g)
R. L.
Asher
,
E. H.
Appelman
, and
B.
Ruscic
,
J. Chem. Phys.
105
,
9781
(
1996
).
27.
(a)
J.-Y.
Roncin
and
F.
Launay
, Atlas of the Vacuum Ultraviolet Emission Spectrum of Molecular Hydrogen, [
J. Phys. Chem. Ref. Data Monogr.
,
4
(
1994
)];
(b)
R. L.
Kelly
, Atomic and Ionic Spectrum Lines Below 2000 Å: Hydrogen Through Krypton, [
J. Phys. Chem. Ref. Data
16, Suppl.
1
(
1987
)].
28.
(a)
E.
Wasserman
,
W. A.
Yager
, and
V. J.
Kuck
,
Chem. Phys. Lett.
7
,
409
(
1970
);
(b)
R. A.
Bernheim
,
H. W.
Bernard
,
P. S.
Wang
,
L. S.
Wood
, and
P. S.
Skell
,
J. Chem. Phys.
53
,
1280
(
1970
);
(c)
E.
Wasserman
,
V. J.
Kuck
,
R. S.
Hutton
, and
W. A.
Yager
,
J. Am. Chem. Soc.
92
,
7491
(
1970
);
(d)
M. D.
Marshall
and
A. R. W.
McKellar
,
J. Chem. Phys.
85
,
3716
(
1986
);
(e)
M.
Rosslein
,
C. M.
Gabrys
,
M.-F.
Jagod
, and
T.
Oka
,
J. Mol. Spectrosc.
153
,
738
(
1992
);
(f)
C. F.
Bender
and
H. F.
Schaefer
 III
,
J. Am. Chem. Soc.
92
,
4984
(
1970
);
(g)
C. F.
Bender
and
H. F.
Schaefer
 III
,
J. Mol. Spectrosc.
37
,
423
(
1971
);
(h)
S. V.
O’Neil
,
H. F.
Schaefer
 III
, and
C. F.
Bender
,
J. Chem. Phys.
55
,
162
(
1971
);
(i)
D. R.
McLaughlin
,
C. F.
Bender
, and
H. F.
Schaefer
III
,
Theor. Chim. Acta
25
,
352
(
1972
).
29.
(a)
J.
Berkowitz
,
J. P.
Greene
,
H.
Cho
, and
B.
Ruscic
,
J. Chem. Phys.
86
,
1235
(
1987
).
(b) In the analogous case of SiH2, which appears to be less reactive than CH2, the singlet and triplet are inverted. In photoionization experiments similar to those presented here, Berkowitz et al. [Ref. 29(a)] have observed ionization from the ground singlet state of SiH2, as well as a rather intense hot band attributable to ionization from the lowest triplet state. Hence the suspicion that wall collisions may not be entirely efficient in electronic relaxation across different multiplicities. In retrospect, rather than attempting to explain the survival mechanism for an enhanced population of the excited triplet, one could simply postulate that the reactive singlet ground state is destroyed by wall collisions more readily than the excited triplet.
30.
F. Westley, D. H. Frizzell, J. T. Herron, R. F. Hampson, and W. G. Mallard, NIST Chemical Kinetics Database, Ver. 6.0 (NIST Std Ref. Database 17) (USGPO, Washington, DC, 1994).
31.
If both appearance energies have thermodynamical significance, then AE0(CH2+/CH4)–AE0(CH2+/CH3)=D0(H3C–H)−D0(H–H)=4.484±0.003 eV (Ref. 25)—4.4781±0.0001 eV (Ref. 9)=0.006±0.003 eV, since AE0(CH2+/CH3)=D0(H2C–H)+IE(CH2) and AE0(CH2+/CH4)=D0(H3C–H)+D0(H2C–H)−D0(H–H)+IE(CH2). The average available internal energy at 298 K of CH3 is slightly larger (1.681kT) than that of CH4 (1.544kT), and hence AE298(CH2+/CH4)–AE298(CH2+/CH3) =0.009±0.003 eV. Furthermore, the CH2+ fragmentation from CH4 is subject to delay from “kinetic shift” (as discussed in Sec. I), which manifests itself as additional roundness in the threshold region. Thus, when the two onsets are compared in practice, the initial ascending portion of the CH2+ fragment yield from CH4 appears to be displaced toward higher energy by ∼0.08 eV, although its slope is such that this gap becomes smaller as the energy increases.
32.
P. C.
Haarhoff
,
Mol. Phys.
7
,
101
(
1963
).
33.
M. E.
Jacox
, Vibrational and Electronic Energy Levels of Polyatomic Transient Molecules, [
J. Phys. Chem. Ref. Data, Monogr.
3
(1994).
34.
The extrapolation leading to IE(CH2) in Ref. 4 is based on a significantly more limited set of data than in the analogous case of CH3 (Ref. 3). There, three distinct Rydberg series (two of which had six members each) were identified and used to obtain IE(CH3). Most members had clearly identifiable and interpretable rotational structure, leading to exact determinations of band origins. Furthermore, there was an even larger body of data on CD3. The extrapolation led to a spectroscopic value which was slightly (∼0.005 eV) higher than the adiabatic IE(CH3) [see Ref. 25 and J. A. Blush, P. Chen, R. T. Wiedman, and M. G. White, J. Chem. Phys. 98, 3557 (1993)]. As opposed to the relative abundance of data on CH3, a single Rydberg series with only four members is known in CH2 (Ref. 4). The first member of the series, which usually displays the greatest departure from constant quantum defect, has been interpreted in terms of its rotational structure (Ref. 3), producing a precise value for the band origin. However, the “first strong band in each group” had to be used in lieu of band origins (Ref. 4) for the three higher members, where exact determinations are more critical. The paucity of data makes the extrapolated IE value somewhat less convincing than in the case of CH3. Herzberg recognizes this fact by assigning a substantially larger error bar to IE(CH2) than to IE(CH3).
35.
Y. Y.
Yamaguchi
and
H. F.
Schaefer
III
,
J. Chem. Phys.
106
,
8753
(
1997
).
36.
K. K.
Irikura
and
J. W.
Hudgens
,
J. Phys. Chem.
96
,
518
(
1997
);
see also
K. K.
Irikura
,
R. D.
Johnson
III
, and
J. W.
Hudgens
,
J. Phys. Chem.
96
,
6131
(
1992
).
37.
(a)
P. R.
Bunker
,
T. J.
Sears
,
A. R. W.
McKellar
,
K. M.
Evenson
, and
F. J.
Lovas
,
J. Chem. Phys.
79
,
1211
(
1983
);
(b)
K. M.
Evenson
,
T. J.
Sears
, and
A. R. W.
McKellar
,
J. Opt. Soc. Am. B
1
,
15
(
1984
).
38.
M.
Rosslein
,
C. M.
Gabrys
,
M.-F.
Jagod
, and
T.
Oka
,
J. Mol. Spectrosc.
153
,
738
(
1992
).
39.
(a) G. Herzberg, Molecular Spectra and Molecular Structure. II. Infrared and Raman Spectra of Polyatomic Molecules (Krieger, Malabar, FL, 1991); (b)
Molecular Spectra and Molecular Structure. III. Electronic Spectra and Electronic Structure of Polyatomic Molecules (Krieger, Malabar, FL, 1991).
40.
H.
Grotheer
,
S.
Kelm
,
H. S. T.
Driver
,
R. J.
Hutcheon
,
R. D.
Lockett
, and
G. N.
Robertson
,
Ber. Bunseges. Phys. Chem.
96
,
1360
(
1992
).
This content is only available via PDF.
You do not currently have access to this content.