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Molecular fragmentations
Journal of Molecular Structure, 67 (1980) 141-150 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
MOLECULAR
FRAGMENTATIONS
Part II**. Structures and energies of molecular formamide and its molecular cation
JAMES R. BEWS and CHRISTOPHER Chemistry Department, (Gt. Britain)
University
fragments
derived from
GLIDEWELL*
of St. Andrews,
St. Andrews,
Fife KY 16 9ST
(Received 6 February 1980)
ABSTRACT Molecular geometries and heats of formation have been calculated, using MLND0/3, for mass spectral fragment pairs (A+ + B) derived from formamide. There are five stable isomeric forms of the molecular ion: [H,NC(OH)]*, (H,NCO)“, [HNC(OH,)]*, [NCH(OH,)]+, and (NCOH,)+ (in order of increasing a@), but no isomer (H,NCHO)+. There are three isomeric forms of (M-H)‘: (H,NCO): (HNCOH)“, and (NCOH,)‘: the only stable form of (M-2H)’ is (NCOH)‘. Other (A/B)’ fragment pairs calculated are (CO/NH,)‘, (HCOINH,)+, (H,O/HCN)+, (H,O/HNC)+, [HO’ + (HCNH)+], and [HO’ + (H,NC)‘]. The structure of the doubly charged ion Mz+ is also reported. INTRODUCTION
In the electron-impact mass spectrum of formamide, the major ions are at m/z 45, RI = lOO% , M’; m/z 44, RI = 27%, (M-H)*; m/z 43, RI = 12% (M--2H)‘; m/z 42, RI = 296, (M-3H)‘; together with ions at m/z 29, RI = 28%; m/z 28, RI = 10% ; and m/i 27, RI = 12% [2]. The ions at m/z 29 and 27 have compositions (HCO)’ and (HCN)‘, respectively, while for m Jz 28 the compositions (CO)‘and (H&N)’ are possible. For the molecular ion M’ at least, the ion can be generated in a 7O-eV electron-impact spectrum in a series of electronic states, as is shown by He(I) photoelectron spectroscopy 131. The fragmentation patterns of the several excited M’ ions will differ, and the various fragment ions may arise from different states of M’: for a given observed fragmentation M++
A++
B
knowledge of the molecular energies of A’and B places restrictions on the state of M’giving rise to these fragments, and in general only the heavier *Author for correspondence. **Ref. 1. 0022-2860/80/0000-0000/$02.25
0 X980 Elsevier Scientific Publishing Company
142
fragments are formed from the electronic ground state of M’. In this paper we deal with the structures and energies of the fragment pairs (A’ + B): in a subsequent paper [4] we shall trace the course of the fragmentation reactions and describe the structures of their transition states. METHOD
Computations were performed on the University of St. Andrews IBM 360/44 computer using the IBM version of MIND013 [ 51. The energy of each fragment was optimized with respect to all geometrical variables, with no assumption of any kind being made. Some fundamental experimental data have been used in association with the computed energies: the dissociation energy of H2 was taken as 36116 cm-’ [6] or 432.05 kJ mol-‘, and the ionization energy of the hydrogen atom was taken as 1312.07 kJ mol-’ [7] _ Point groups, molecular states, and molecular energies are given in Table 1, and optimized molecular geometries in Table 2. Energies of fragment pairs (A* + B) are given in Fig. 1. Ionization energies of neutral diamagnetic species are in Table 3. RESULTS
AND DISCUSSION
Molecular strut tures The parent neutral molecuIe is calculated to be completely $anar. An early microwave study [S] found the molecule to be planar, but in a iater investigation [ 91 the sum of angles at nitrogen was found to be 356.65”, with a barrier to pyramidal inversion at nitrogen of 370 + 50 cm-’ (i.e. 4.4 + 0.6 kJ mol-I). The calculated valence shell ionization energies and the corresponding electronic states of (HiNCHO)’ are given in Table 4: ordering of the states is in agreement with that derived from ab initio calculations using experimental geometries [lo] but our energies are closer to those observed experimentally [3] than those of the earlier calculations. The highest valence shell ionization energy is calculated to be 32.57 eV: with the values of A@ for formamide, and the enthalpies of atomization of the constituent elements, it may be calculated that the energy required for complete atomization H,NCHO*
3H+C+N+O
is 23.91 eV, while for atomization and.concurrent formation of a single cation, the minimum energy required, corresponding to ionization of carbon H,NCHO-+3H+N+O+C+ is 35.19 eV. Consequently it may be anticipated that ionization of the deeper valence shell electrons will give rise to extensive decomposition: for most small molecules, 70-eV electrons are adequate to cause complete atomization.
143 TABLE
1
Molecular energies for optimized
geometries
Molecule
Point group
State
H,NCHO
CS
‘A’ 2A 2 Av
CH,NC(OH)I+ (H,NCO)+ W’WOH,)I+
PCWOH,)l+
(NCOH,)+ (H,NCO)+ (HNCOH)’ [H,NC(OH) (NCOH,)+ HNCO HOCN (HOCN)’ (NCO)’ (HCNH)+ (H,NC)+ HNC HCN (HNC)+ (HCN)’
C,
CS
c,
2A
C 3V
2A, ‘A
2
A*
C, c Cl Cl
2V
j 2+
1
'A 'A 'A I A 1’ 'A'
C 2”
CS CS CS
,A”
C OOV C OOV C 2V
‘x+ lx+ ‘A, ‘x+ ‘z+ 2 *
A
=A’
NH, (NH 3)’ NH, (NH,)+ (NH,)’
‘A , =A: 2B, ‘B, ‘A,
Total
energy (ev)
-675.4934 -667.4779 -667.0462 -667.0416 -665.2774 -664.8873 -652.7633 -652.6766 -652.3662
-651.6991 -645.6322 -645.2275 -635.1328 -617.7211 -340.1381 -338.8489 -333.2270 -332.5656 -322.4655 -322.2493 -237.1095 -227.8067 -220.6344 -210.7071 -209.3704
Afl
(kJ mol-‘)
-217.81 + 555.59 + 597.23 + 597.68 + 767.90 + 805.54 + 550.79 -i-559.15 + 2013.67 + 653.46 -185.72 -146.67 + 827.34 + 1082.78 + 837.60 + 961.99 + 79.87 + 143.68 +1118.21 + 1139.07 -39.23 + 858.38 + 125.84 + 1083.70 +1212.67
The energies in Table 4 are vertical ionization energies: our version of MIND0/3 is unable to handle the excited doublet states arising from ionization of any but the least bound electrons; consequently we have been able to calculate an adiabatic ionization energy only for the Z%state. However, vertical ionization energies give an adequate indication of the energies of the excited states of M+. There is no reason why the geometry of M* should necessarily be closely similar to that of the neutral M, and for formamide there are seven plausible constitutions for M’ (I-VII):
(I) [
H,N-Cp".l'
(VI
W)
[N-C<;‘]’ WI)
(III) [N-C-0$]
(Jm
(IV)
144 TABLE
2
Molecular geometries at equilibrium H
H,
“N-C< /
0
Hb
H a;N-C Hb
‘OH,
+ 1
H,N, 1.016 A; HbN, 1.016 A; NC, 1.334 A; CH,, 1.140 A; Co, 1.207 A; H,NH,, 110.4O; H,NC, 123.9”; &NC, 125.7”; NCH,, 110.0” ; NCO, 126.4”; H&O, 123.4"; whole molecule planar H,N, 1.020 A; HbN, 1.031 A; NC, 1.281 A; CO, 1.209 A; OH,, 0.958 A; H,NH,, 111.3”; H,NC, 121.9”; H,NC, 125.3”; NCO, 145.4”; COH,, 139.4”; 6(H,OCN), 222.4”; S(H,NCO), 182.2”; B(HbNCO), 17.4” HN, 1.046 A; NC, 1.446 A;CO, 1.140 143.2”; 6(HNCO), 60°, 180°, 300”
(H,NCO)+
A; HNC, 113.6”;
NCO,
H,N, 1.004 A; NC, 1.160 A;CO, 1.362 A; OH,, 0.979 A; OH,, 0.973 A; H,NC, 165.3”; NCO, 152.9”; COH,, 126.5”; COH,, i24.7”; HbOH,, 108.3”; s(H,NCO), 179.5” (see text)
[ Ha,N-C”::]
NC, 1.206 A; CH,, 1.140 A; Co, 1.414 A; OH,=OH,, 0.966 A; NCH,, 118_4”;NCO, 129.1”; COHb=COH,, 120.5”; s(NCHO), 180.0”; 6(HbOCN), 112.5”; 6(H,OCN), 247.5”
WCOH,)’
(C,,)
NC, 1.139
A;CO,
1.451
(H.NCO)+
CC,,)
HN,1.016
a;NC,
1.281 A;CO,1.141
(H,NCOHb)+
A;COH,
114.3”
A;HNC,122.2" H,NC,
H,N,1.003 A;NC,1.173 A;CO,1.249 A;OHb,0.960A; 144.5”; NCO, 158.4”; CO&, 116.0”; 6(H,NCO), 157.9”; s(HbOCN), 109.5” 2+
H +N-C Hb/
A; OH, 1.019
'OH,
(NCOH,)’
I
H,N,1.036 A;HbN,1.035A;NC,1.254 A;Co,l.l82A; OH,, 0.977 A; H,NHb, 114.1O; H,NC, 122.0”; HbNC, 122.5”; NCO, 175.3”; COH,, 134.4”; s(H,NCO), -41.2”; 6(HbNCO), 124.5”; h(H,OCN), 143.4” A; OH, 0.957
A; COH, 124.7O
HNCO
HN, 1.002 A; NC, 1.211 A; CO, 1.182 162.8”; s(HNCO), 180.0”
A; HNC, 134.1”;
NCO,
HOCN
HO, 0.951 A; OC, 1.293 A; CN, 1.160 174.8“; 6(HOCN), 180.0”
A; HOC, 113.6”:
OCN,
(HOCN)’
HO, 0.959 A; OC, 1.221 A;CN, 1.215 A; HOC, 125.9”; 172.1”; s(HOCN), 180.0“ OC, 1.136 A; CN, 1.312 A; OCN, 180.0”
OCN,
CC,“)
(OCN)’
NC, 1.140
A; CO, 1.347
H,C, 1.089 A; CN, 1.138 CNHb, 180.0”
(HaCNHb )’ l
A; NHb, 0.997
A; H,CN,
H,N, 1.034 A;H,N,1.034 A;NC,1.223 A;HaNHb, H,NC, 126.7” ; HbNC, 126.7”; whole ion planar
HNC
HN, 1.013
A; NC, 1.145
A; HNC, 180.0”
HCN
HC, 1.096
A; CN, 1.152
A; HCN, 180.0”
180.0”; 106.6";
145 TABLE
2 (continued)
(HNC)’
HN, 1.027
A; NC, 1.158
a; HNC, 134.4”
(HCN)’
HC, 1.103
/k; CN, 1.122
a; HCN, 173.9”
NH,
NH, 1.031
A; HNH, 104.3’;
molecule has C,, symmetry
(NH,)+
NH, 1.012
R; HNH, 120.0°;
ion has DJh symmetry
NH*
NH, 1.036
A; HNH,
molecule has C,, symmetry
102.2”;
(NH, )+
(‘B,)
NH, 1.000
A; HNH, 140.6”;
ion has C,, symmetry
(NH,)’
(‘A,)
NH, 1.036
A; HNH, 106.0”;
ion has C,, symmetry
Of these, (I) requires no hydrogen migration after ionization of the neutral molecule; (II), (III), and (IV) each require migration of one hydrogen atom; (V) and (VI) require migration of two hydrogens; and (VII) requires migration of all three hydrogens. Optimizations were performed with input geometries corresponding to each of the constitutions (I-VII). No bound solution was found for (I), but (II), (IV), (V), (VI), and (VII) all gave minima, which in order of increasing A@ are (II) < (IV) - (V) < (VI) < (VII) (cf. Table 1 and Fig. 1): (III) has no minimum but converges to (V). It is noteworthy that for (I), the most obvious constitution given the constitution of the neutral molecule, no bound state could be found. No dihedral angles for the OH, hydrogens are quoted for (V) in Table 2 and, since there is almost free rotation (barrier < 1 kJ mol-‘) about the CO bond in this ion, the CO bond is quite strong as A@ for the dissociation [HNC(OH,)I
+ + (HNC)++
Hz0
is 296.2 kJ mol-‘. We shall discuss the equilibria between these five isomers of M’elsewhere [4]. The structures are shown in Fig. 2. We briefly investigated the structure of M2*: the lowest energy state corresponds to doubly (N,O) protonated HNCO: the structure is in Fig. 3. The mass spectral peak at m/z 44 corresponds to (M-H)’ for which there are five plausible constitutions (VIII-XII):
(XII) Constitutions (VIII), (IX) and (X) correspond to N-, C-, and 0-protonated HNCO, and (X), (XI), and (XII) correspond to N-, C-, and 0-protonated HOCN: optimizations were performed with each constitution as input.
146
M
M’
(M-H)’
(M;ttI+
1co;mJ31+
lH2;;;CNl+
(M-3H)+
[HCO/NH2]+
WH2CNI’
NEUTRAL FRAGMENTS
i lH2NCHO)’ 1831~1 (NCOI*+
3H’
I7309
z (H2NCHO)* 1L.u 0 fNCOt*H2 -H’ 1298 8 (NCOH)’ * 2H’ 1259 1.
fi lH2NCHOl*
HCO .iNH$*I’A,l
12211 r lH2NCHOl’ 1172L
1202 0 lH2Ol‘ . HCN 1112f. %iiF HO’.(H2NCI* 1032 6
6 IHzNCHO)’ 1000 1
ICOI'. NH3 9L6 5 HO’.
IHCNH)’
“J?%NCS 8939
(HNCOHl*. H’
INC;:og”2”’
s 766 8
I HNC10H211C
HCO’
‘W-tlCtll IE 0 Lo2ev
I
Adlabatlc
l
NH2
l115 2
I E. 8 016 eV
HCN
c Hz0
Cl?
kHj
HzNCHO
-2178
Fig. 1. Energies
of the mass spectral
fragment
pairs, and of some
neutral
fragment
pairs.
147 TABLE
3
Calculated ionization energies and dipole momenki Molecule
Vertical IE (eV)
H,NCHO HNCO HOCN HCN HNC NH,
10.40 10.36 10.61 10.50 11.27 10.06
aD = 3.3356 x lo-““C TABLE
Adiabatic
m (eV)
8.02
D.p.m.
(D)a
3.87 2.82 2.62 1.63 2.23 1.63
b
10.10 10.32 10.76 9.30
m . bNo bound state found for (HNCO)+.
4
Energies and molecular states of (H,NCHO)’ State
Vertical IE (eV)
Al-$+ (kJ mol-*)
+ 785.82 + 794.79 f 1000.12 + 1172.36 + 1221.09 + 1440.01 + 1831.06 + 2456.15 + 2924.94 -217.81.
10.402 10.495 12.623 14.408 14.913 17.182 21.235 27.713 32.572 Ground stake of molecule
IBound solutions were found corresponding to (VIII), (X) and (XII), and these structures are shown in Fig. 4: no stable minima were found for (IX) to (VIII) and (XI) to (XII). The stable isomers are or (XI), (IX) converging in order of increasing A_@: (VIH) < (X) < (XI). We shall discuss eIsewhere 141, the equilibria between the isomers of (M-H)’ and their formation from the various isomers of M”. For the mass spectral peak at m/z 43, (M-ZH)+, three constitutions are plausible (XIII-XV), corresponding to N-, C-, and O-protonation, respectively, of the NC0 radicaI: [H’hl-C-Ol’ (xrrr)
E;C-Oj+ WW
[N-C-O,A
(XV)
Only for (XV) was a bound solution found: like its neutral analogue, (HOCN)’ is planar with LOCN close to 180”.
148
Fig. 2. Structures of the isomers of M’: (a) (II), (b) (IV), (c) (V), (d) (VI), (e) (VII).
Fig. 3. Structure of M2’.
$-o N
c
0
a
Fig. 4. Structures of the isomers of (M-I-I)+:
(a) (VIII),
(b) (X), (c) (XII).
Of the lighter fragment ions (HCO)‘, m/z 29, is linear as previously described [l] : for the ion of composition (HCN)’ at m/z 27, two constitutions are possible, (HCN)’ and (HNC)‘. For the ion with m/z 28, two compositions (CO)‘and (H&N)‘are possible, and for the latter three constitutions are plausible (XVI-XVIII): [ I$C--Nlf (XVI)
[H-C-N-~] (XVII)
[C-N<] (XVIII)
(XVI) and (XVII) are, respectively, C- and N-protonated HCN, while (XVII) and (XVIII) are C- and AT-protonated HNC. No minimum was found for
149
(XVI) which converges to the linear ion (XVII), which is more stable than (XVIII) by some 124 kJ mol-‘. Under high resolution (MS-902) the peak at m/z 28 appears as two components: accurate mass measurement was not possible, but the accurate mass ratio was determined as 1.000835. Calculated ratios are: (H2CN)‘/(NZ)+, 1.000449; (N,)‘/(CO)‘, 1.000401; (H&N)‘/(CO)‘, 1.000850: these data strongly suggest that the two components of m/z 28 are (I-I&N)’ and (CO)‘. Further evidence for the absence of (N,)’ from m/z 28 is found in the observation that in the spectrum of formamide, m/z 32 corresponding to (0,)’ is virtually absent, while (O,)* and (N2)+ have approximately equal intensities in a background spectrum. Of the two most plausible fragment pairs derived from isomer (IV) of M+, (H3NCO)‘, namely (CO)’ + NH3 and CO + (NH,)*, the second is lower in energy by some 145 kJ mol-‘: the ion (NH,)* is calculated to be rigorously planar.
Fragment
energies
The energies of the fragment pairs (A’ + B), together with those of a few pairs of neutral fragments are shown in Fig. 1. The charged fragment pairs include all the major ions observed in the mass spectrum: it is assumed that once the parent molecule has been vertically ionised to one of the excited states of M’, either in a photoelectron spectrometer or in a mass spectrometer, that no further energization of that ion can occur. It is then apparent from Fig. 1 that four isomers of M’ ((II), (IV), (V) and (VI)) and two isomers of (M-H)’ ((VIII) and (X)) are accessible from the ground state X(H,NCHO), but that (NCOH3)’ and (NCOH*)’ are accessible only from the e and higher states of (H,NCHO)‘. In a similar way, (NCOH)‘, (HCO)‘, (CO)‘, (HCNH)‘_and (HCN)’ are accessible from B or higher states only, (H,NC)’ from C or higher states only, and (NCO)’ from l? or higher states only: unfortunately, the chargeexchange mass spectrum of H,NCHO, which would provide direct experimental confirmation of these results, does not appear to have been determined. Some bond dissociation energies are in Table 5, and proton affinities in Table 6. In Table 7 are collected together the calculated appearance potenTABLE
5
Bond dissociation energies (kJ mol-‘) D( H,N-CHO) D( H,N-CHO)’ D(H,N--CO)+ D( HNC-OH,)+ D(NCH-OH,)+ D( H,N-CO)+
333.0 342.4 204.6 296.2 146.9 475.3
150 TABLE
6
Proton affinities
(kJ mol-’ )
Molecule
P
HNCO
792 783 728 822
HNCU HOCN HOCN
TABLE
Molecule
P (kJ mol-‘)
HCN WC HNC i-I,
834 646 770 517
7
Appearance potentials of fragment ions Ion
m/z
AP (ev)
Ion
m/z
@ (@VI
(H,NCO)+ (HNCOH)+ (NCOH,)’ (NCOH)+ (NCO)’ (HCO)+ (CO 1’
44 44 44 43 42 29 28
10.20 10.29 11.27 10.83 15.72 11.57 12.07
(HCNH)* (H,NC)+ (HNC)’ (HCN)’ (H,O)+ (NH,)+ (NH,)’
28 28 27 27 18 17 16
11.67 12.96 11.52 11.74 13.13 10.57 13.34
tials of the fragmentations: only for m/z 44 has an experimental value been reported [II] : 10.24 eV, in excellent agreement with the values calculated for the isomers (H,NCO)’ and (HNCOH)‘. REFERENCES 1 J. R. Bews, C. Glidewell and P. H. Vidaud, J. Mol. Struct., 64 (1980) 75. (Part I). 2 J. A. Gilpin, Anal. Chem., 31 (1959) 4404. 3 D. W. Turner, C. Baker, A. D. Baker and C. R. Brundle, Molecular Photoelectron Spectroscopy, Wiley-Interscience, London, 1970. 4 J. R. Bews and C. Glidewell, 67 (1980) 151. (Part III). 5 R. C. Bingham, M. J. S. Dewar and D. H. Lo, J. Am. Chem. Sot., 97 (1975) 1285; QCPE, 10 (1976) 309. 6 G. Herzberg, Spectra of Diatomic Molecules, 2nd edn., Van Nostrand, Princeton, NJ, 1950. 7 B. N. Taylor, W. H. Parker and D. N. Langenberg, Rev. Mod. Phys., 41(1969) 477. 8 R. J. Kurland and E. B. Wilson, Jr., J. Chem. Phys., 27 (1957) 585. 9 C. Costain and J. M. Dowling, J. Chem. Phys., 32 (1960) 158. 10 H. Basch, M. B. Robin and N. A. Kuebler, J. Chem. Phys., 49 (1968) 5007. 11 H. M. Rosenstock, K. Draxl, B. W. Steiner and J. T. Herron, Energetics of Gaseous Ions, National Bureau of Standards, Washington, DC, 1977.
MOLECULAR
FRAGMENTATIONS
Part II**. Structures and energies of molecular formamide and its molecular cation
JAMES R. BEWS and CHRISTOPHER Chemistry Department, (Gt. Britain)
University
fragments
derived from
GLIDEWELL*
of St. Andrews,
St. Andrews,
Fife KY 16 9ST
(Received 6 February 1980)
ABSTRACT Molecular geometries and heats of formation have been calculated, using MLND0/3, for mass spectral fragment pairs (A+ + B) derived from formamide. There are five stable isomeric forms of the molecular ion: [H,NC(OH)]*, (H,NCO)“, [HNC(OH,)]*, [NCH(OH,)]+, and (NCOH,)+ (in order of increasing a@), but no isomer (H,NCHO)+. There are three isomeric forms of (M-H)‘: (H,NCO): (HNCOH)“, and (NCOH,)‘: the only stable form of (M-2H)’ is (NCOH)‘. Other (A/B)’ fragment pairs calculated are (CO/NH,)‘, (HCOINH,)+, (H,O/HCN)+, (H,O/HNC)+, [HO’ + (HCNH)+], and [HO’ + (H,NC)‘]. The structure of the doubly charged ion Mz+ is also reported. INTRODUCTION
In the electron-impact mass spectrum of formamide, the major ions are at m/z 45, RI = lOO% , M’; m/z 44, RI = 27%, (M-H)*; m/z 43, RI = 12% (M--2H)‘; m/z 42, RI = 296, (M-3H)‘; together with ions at m/z 29, RI = 28%; m/z 28, RI = 10% ; and m/i 27, RI = 12% [2]. The ions at m/z 29 and 27 have compositions (HCO)’ and (HCN)‘, respectively, while for m Jz 28 the compositions (CO)‘and (H&N)’ are possible. For the molecular ion M’ at least, the ion can be generated in a 7O-eV electron-impact spectrum in a series of electronic states, as is shown by He(I) photoelectron spectroscopy 131. The fragmentation patterns of the several excited M’ ions will differ, and the various fragment ions may arise from different states of M’: for a given observed fragmentation M++
A++
B
knowledge of the molecular energies of A’and B places restrictions on the state of M’giving rise to these fragments, and in general only the heavier *Author for correspondence. **Ref. 1. 0022-2860/80/0000-0000/$02.25
0 X980 Elsevier Scientific Publishing Company
142
fragments are formed from the electronic ground state of M’. In this paper we deal with the structures and energies of the fragment pairs (A’ + B): in a subsequent paper [4] we shall trace the course of the fragmentation reactions and describe the structures of their transition states. METHOD
Computations were performed on the University of St. Andrews IBM 360/44 computer using the IBM version of MIND013 [ 51. The energy of each fragment was optimized with respect to all geometrical variables, with no assumption of any kind being made. Some fundamental experimental data have been used in association with the computed energies: the dissociation energy of H2 was taken as 36116 cm-’ [6] or 432.05 kJ mol-‘, and the ionization energy of the hydrogen atom was taken as 1312.07 kJ mol-’ [7] _ Point groups, molecular states, and molecular energies are given in Table 1, and optimized molecular geometries in Table 2. Energies of fragment pairs (A* + B) are given in Fig. 1. Ionization energies of neutral diamagnetic species are in Table 3. RESULTS
AND DISCUSSION
Molecular strut tures The parent neutral molecuIe is calculated to be completely $anar. An early microwave study [S] found the molecule to be planar, but in a iater investigation [ 91 the sum of angles at nitrogen was found to be 356.65”, with a barrier to pyramidal inversion at nitrogen of 370 + 50 cm-’ (i.e. 4.4 + 0.6 kJ mol-I). The calculated valence shell ionization energies and the corresponding electronic states of (HiNCHO)’ are given in Table 4: ordering of the states is in agreement with that derived from ab initio calculations using experimental geometries [lo] but our energies are closer to those observed experimentally [3] than those of the earlier calculations. The highest valence shell ionization energy is calculated to be 32.57 eV: with the values of A@ for formamide, and the enthalpies of atomization of the constituent elements, it may be calculated that the energy required for complete atomization H,NCHO*
3H+C+N+O
is 23.91 eV, while for atomization and.concurrent formation of a single cation, the minimum energy required, corresponding to ionization of carbon H,NCHO-+3H+N+O+C+ is 35.19 eV. Consequently it may be anticipated that ionization of the deeper valence shell electrons will give rise to extensive decomposition: for most small molecules, 70-eV electrons are adequate to cause complete atomization.
143 TABLE
1
Molecular energies for optimized
geometries
Molecule
Point group
State
H,NCHO
CS
‘A’ 2A 2 Av
CH,NC(OH)I+ (H,NCO)+ W’WOH,)I+
PCWOH,)l+
(NCOH,)+ (H,NCO)+ (HNCOH)’ [H,NC(OH) (NCOH,)+ HNCO HOCN (HOCN)’ (NCO)’ (HCNH)+ (H,NC)+ HNC HCN (HNC)+ (HCN)’
C,
CS
c,
2A
C 3V
2A, ‘A
2
A*
C, c Cl Cl
2V
j 2+
1
'A 'A 'A I A 1’ 'A'
C 2”
CS CS CS
,A”
C OOV C OOV C 2V
‘x+ lx+ ‘A, ‘x+ ‘z+ 2 *
A
=A’
NH, (NH 3)’ NH, (NH,)+ (NH,)’
‘A , =A: 2B, ‘B, ‘A,
Total
energy (ev)
-675.4934 -667.4779 -667.0462 -667.0416 -665.2774 -664.8873 -652.7633 -652.6766 -652.3662
-651.6991 -645.6322 -645.2275 -635.1328 -617.7211 -340.1381 -338.8489 -333.2270 -332.5656 -322.4655 -322.2493 -237.1095 -227.8067 -220.6344 -210.7071 -209.3704
Afl
(kJ mol-‘)
-217.81 + 555.59 + 597.23 + 597.68 + 767.90 + 805.54 + 550.79 -i-559.15 + 2013.67 + 653.46 -185.72 -146.67 + 827.34 + 1082.78 + 837.60 + 961.99 + 79.87 + 143.68 +1118.21 + 1139.07 -39.23 + 858.38 + 125.84 + 1083.70 +1212.67
The energies in Table 4 are vertical ionization energies: our version of MIND0/3 is unable to handle the excited doublet states arising from ionization of any but the least bound electrons; consequently we have been able to calculate an adiabatic ionization energy only for the Z%state. However, vertical ionization energies give an adequate indication of the energies of the excited states of M+. There is no reason why the geometry of M* should necessarily be closely similar to that of the neutral M, and for formamide there are seven plausible constitutions for M’ (I-VII):
(I) [
H,N-Cp".l'
(VI
W)
[N-C<;‘]’ WI)
(III) [N-C-0$]
(Jm
(IV)
144 TABLE
2
Molecular geometries at equilibrium H
H,
“N-C< /
0
Hb
H a;N-C Hb
‘OH,
+ 1
H,N, 1.016 A; HbN, 1.016 A; NC, 1.334 A; CH,, 1.140 A; Co, 1.207 A; H,NH,, 110.4O; H,NC, 123.9”; &NC, 125.7”; NCH,, 110.0” ; NCO, 126.4”; H&O, 123.4"; whole molecule planar H,N, 1.020 A; HbN, 1.031 A; NC, 1.281 A; CO, 1.209 A; OH,, 0.958 A; H,NH,, 111.3”; H,NC, 121.9”; H,NC, 125.3”; NCO, 145.4”; COH,, 139.4”; 6(H,OCN), 222.4”; S(H,NCO), 182.2”; B(HbNCO), 17.4” HN, 1.046 A; NC, 1.446 A;CO, 1.140 143.2”; 6(HNCO), 60°, 180°, 300”
(H,NCO)+
A; HNC, 113.6”;
NCO,
H,N, 1.004 A; NC, 1.160 A;CO, 1.362 A; OH,, 0.979 A; OH,, 0.973 A; H,NC, 165.3”; NCO, 152.9”; COH,, 126.5”; COH,, i24.7”; HbOH,, 108.3”; s(H,NCO), 179.5” (see text)
[ Ha,N-C”::]
NC, 1.206 A; CH,, 1.140 A; Co, 1.414 A; OH,=OH,, 0.966 A; NCH,, 118_4”;NCO, 129.1”; COHb=COH,, 120.5”; s(NCHO), 180.0”; 6(HbOCN), 112.5”; 6(H,OCN), 247.5”
WCOH,)’
(C,,)
NC, 1.139
A;CO,
1.451
(H.NCO)+
CC,,)
HN,1.016
a;NC,
1.281 A;CO,1.141
(H,NCOHb)+
A;COH,
114.3”
A;HNC,122.2" H,NC,
H,N,1.003 A;NC,1.173 A;CO,1.249 A;OHb,0.960A; 144.5”; NCO, 158.4”; CO&, 116.0”; 6(H,NCO), 157.9”; s(HbOCN), 109.5” 2+
H +N-C Hb/
A; OH, 1.019
'OH,
(NCOH,)’
I
H,N,1.036 A;HbN,1.035A;NC,1.254 A;Co,l.l82A; OH,, 0.977 A; H,NHb, 114.1O; H,NC, 122.0”; HbNC, 122.5”; NCO, 175.3”; COH,, 134.4”; s(H,NCO), -41.2”; 6(HbNCO), 124.5”; h(H,OCN), 143.4” A; OH, 0.957
A; COH, 124.7O
HNCO
HN, 1.002 A; NC, 1.211 A; CO, 1.182 162.8”; s(HNCO), 180.0”
A; HNC, 134.1”;
NCO,
HOCN
HO, 0.951 A; OC, 1.293 A; CN, 1.160 174.8“; 6(HOCN), 180.0”
A; HOC, 113.6”:
OCN,
(HOCN)’
HO, 0.959 A; OC, 1.221 A;CN, 1.215 A; HOC, 125.9”; 172.1”; s(HOCN), 180.0“ OC, 1.136 A; CN, 1.312 A; OCN, 180.0”
OCN,
CC,“)
(OCN)’
NC, 1.140
A; CO, 1.347
H,C, 1.089 A; CN, 1.138 CNHb, 180.0”
(HaCNHb )’ l
A; NHb, 0.997
A; H,CN,
H,N, 1.034 A;H,N,1.034 A;NC,1.223 A;HaNHb, H,NC, 126.7” ; HbNC, 126.7”; whole ion planar
HNC
HN, 1.013
A; NC, 1.145
A; HNC, 180.0”
HCN
HC, 1.096
A; CN, 1.152
A; HCN, 180.0”
180.0”; 106.6";
145 TABLE
2 (continued)
(HNC)’
HN, 1.027
A; NC, 1.158
a; HNC, 134.4”
(HCN)’
HC, 1.103
/k; CN, 1.122
a; HCN, 173.9”
NH,
NH, 1.031
A; HNH, 104.3’;
molecule has C,, symmetry
(NH,)+
NH, 1.012
R; HNH, 120.0°;
ion has DJh symmetry
NH*
NH, 1.036
A; HNH,
molecule has C,, symmetry
102.2”;
(NH, )+
(‘B,)
NH, 1.000
A; HNH, 140.6”;
ion has C,, symmetry
(NH,)’
(‘A,)
NH, 1.036
A; HNH, 106.0”;
ion has C,, symmetry
Of these, (I) requires no hydrogen migration after ionization of the neutral molecule; (II), (III), and (IV) each require migration of one hydrogen atom; (V) and (VI) require migration of two hydrogens; and (VII) requires migration of all three hydrogens. Optimizations were performed with input geometries corresponding to each of the constitutions (I-VII). No bound solution was found for (I), but (II), (IV), (V), (VI), and (VII) all gave minima, which in order of increasing A@ are (II) < (IV) - (V) < (VI) < (VII) (cf. Table 1 and Fig. 1): (III) has no minimum but converges to (V). It is noteworthy that for (I), the most obvious constitution given the constitution of the neutral molecule, no bound state could be found. No dihedral angles for the OH, hydrogens are quoted for (V) in Table 2 and, since there is almost free rotation (barrier < 1 kJ mol-‘) about the CO bond in this ion, the CO bond is quite strong as A@ for the dissociation [HNC(OH,)I
+ + (HNC)++
Hz0
is 296.2 kJ mol-‘. We shall discuss the equilibria between these five isomers of M’elsewhere [4]. The structures are shown in Fig. 2. We briefly investigated the structure of M2*: the lowest energy state corresponds to doubly (N,O) protonated HNCO: the structure is in Fig. 3. The mass spectral peak at m/z 44 corresponds to (M-H)’ for which there are five plausible constitutions (VIII-XII):
(XII) Constitutions (VIII), (IX) and (X) correspond to N-, C-, and 0-protonated HNCO, and (X), (XI), and (XII) correspond to N-, C-, and 0-protonated HOCN: optimizations were performed with each constitution as input.
146
M
M’
(M-H)’
(M;ttI+
1co;mJ31+
lH2;;;CNl+
(M-3H)+
[HCO/NH2]+
WH2CNI’
NEUTRAL FRAGMENTS
i lH2NCHO)’ 1831~1 (NCOI*+
3H’
I7309
z (H2NCHO)* 1L.u 0 fNCOt*H2 -H’ 1298 8 (NCOH)’ * 2H’ 1259 1.
fi lH2NCHOl*
HCO .iNH$*I’A,l
12211 r lH2NCHOl’ 1172L
1202 0 lH2Ol‘ . HCN 1112f. %iiF HO’.(H2NCI* 1032 6
6 IHzNCHO)’ 1000 1
ICOI'. NH3 9L6 5 HO’.
IHCNH)’
“J?%NCS 8939
(HNCOHl*. H’
INC;:og”2”’
s 766 8
I HNC10H211C
HCO’
‘W-tlCtll IE 0 Lo2ev
I
Adlabatlc
l
NH2
l115 2
I E. 8 016 eV
HCN
c Hz0
Cl?
kHj
HzNCHO
-2178
Fig. 1. Energies
of the mass spectral
fragment
pairs, and of some
neutral
fragment
pairs.
147 TABLE
3
Calculated ionization energies and dipole momenki Molecule
Vertical IE (eV)
H,NCHO HNCO HOCN HCN HNC NH,
10.40 10.36 10.61 10.50 11.27 10.06
aD = 3.3356 x lo-““C TABLE
Adiabatic
m (eV)
8.02
D.p.m.
(D)a
3.87 2.82 2.62 1.63 2.23 1.63
b
10.10 10.32 10.76 9.30
m . bNo bound state found for (HNCO)+.
4
Energies and molecular states of (H,NCHO)’ State
Vertical IE (eV)
Al-$+ (kJ mol-*)
+ 785.82 + 794.79 f 1000.12 + 1172.36 + 1221.09 + 1440.01 + 1831.06 + 2456.15 + 2924.94 -217.81.
10.402 10.495 12.623 14.408 14.913 17.182 21.235 27.713 32.572 Ground stake of molecule
IBound solutions were found corresponding to (VIII), (X) and (XII), and these structures are shown in Fig. 4: no stable minima were found for (IX) to (VIII) and (XI) to (XII). The stable isomers are or (XI), (IX) converging in order of increasing A_@: (VIH) < (X) < (XI). We shall discuss eIsewhere 141, the equilibria between the isomers of (M-H)’ and their formation from the various isomers of M”. For the mass spectral peak at m/z 43, (M-ZH)+, three constitutions are plausible (XIII-XV), corresponding to N-, C-, and O-protonation, respectively, of the NC0 radicaI: [H’hl-C-Ol’ (xrrr)
E;C-Oj+ WW
[N-C-O,A
(XV)
Only for (XV) was a bound solution found: like its neutral analogue, (HOCN)’ is planar with LOCN close to 180”.
148
Fig. 2. Structures of the isomers of M’: (a) (II), (b) (IV), (c) (V), (d) (VI), (e) (VII).
Fig. 3. Structure of M2’.
$-o N
c
0
a
Fig. 4. Structures of the isomers of (M-I-I)+:
(a) (VIII),
(b) (X), (c) (XII).
Of the lighter fragment ions (HCO)‘, m/z 29, is linear as previously described [l] : for the ion of composition (HCN)’ at m/z 27, two constitutions are possible, (HCN)’ and (HNC)‘. For the ion with m/z 28, two compositions (CO)‘and (H&N)‘are possible, and for the latter three constitutions are plausible (XVI-XVIII): [ I$C--Nlf (XVI)
[H-C-N-~] (XVII)
[C-N<] (XVIII)
(XVI) and (XVII) are, respectively, C- and N-protonated HCN, while (XVII) and (XVIII) are C- and AT-protonated HNC. No minimum was found for
149
(XVI) which converges to the linear ion (XVII), which is more stable than (XVIII) by some 124 kJ mol-‘. Under high resolution (MS-902) the peak at m/z 28 appears as two components: accurate mass measurement was not possible, but the accurate mass ratio was determined as 1.000835. Calculated ratios are: (H2CN)‘/(NZ)+, 1.000449; (N,)‘/(CO)‘, 1.000401; (H&N)‘/(CO)‘, 1.000850: these data strongly suggest that the two components of m/z 28 are (I-I&N)’ and (CO)‘. Further evidence for the absence of (N,)’ from m/z 28 is found in the observation that in the spectrum of formamide, m/z 32 corresponding to (0,)’ is virtually absent, while (O,)* and (N2)+ have approximately equal intensities in a background spectrum. Of the two most plausible fragment pairs derived from isomer (IV) of M+, (H3NCO)‘, namely (CO)’ + NH3 and CO + (NH,)*, the second is lower in energy by some 145 kJ mol-‘: the ion (NH,)* is calculated to be rigorously planar.
Fragment
energies
The energies of the fragment pairs (A’ + B), together with those of a few pairs of neutral fragments are shown in Fig. 1. The charged fragment pairs include all the major ions observed in the mass spectrum: it is assumed that once the parent molecule has been vertically ionised to one of the excited states of M’, either in a photoelectron spectrometer or in a mass spectrometer, that no further energization of that ion can occur. It is then apparent from Fig. 1 that four isomers of M’ ((II), (IV), (V) and (VI)) and two isomers of (M-H)’ ((VIII) and (X)) are accessible from the ground state X(H,NCHO), but that (NCOH3)’ and (NCOH*)’ are accessible only from the e and higher states of (H,NCHO)‘. In a similar way, (NCOH)‘, (HCO)‘, (CO)‘, (HCNH)‘_and (HCN)’ are accessible from B or higher states only, (H,NC)’ from C or higher states only, and (NCO)’ from l? or higher states only: unfortunately, the chargeexchange mass spectrum of H,NCHO, which would provide direct experimental confirmation of these results, does not appear to have been determined. Some bond dissociation energies are in Table 5, and proton affinities in Table 6. In Table 7 are collected together the calculated appearance potenTABLE
5
Bond dissociation energies (kJ mol-‘) D( H,N-CHO) D( H,N-CHO)’ D(H,N--CO)+ D( HNC-OH,)+ D(NCH-OH,)+ D( H,N-CO)+
333.0 342.4 204.6 296.2 146.9 475.3
150 TABLE
6
Proton affinities
(kJ mol-’ )
Molecule
P
HNCO
792 783 728 822
HNCU HOCN HOCN
TABLE
Molecule
P (kJ mol-‘)
HCN WC HNC i-I,
834 646 770 517
7
Appearance potentials of fragment ions Ion
m/z
AP (ev)
Ion
m/z
@ (@VI
(H,NCO)+ (HNCOH)+ (NCOH,)’ (NCOH)+ (NCO)’ (HCO)+ (CO 1’
44 44 44 43 42 29 28
10.20 10.29 11.27 10.83 15.72 11.57 12.07
(HCNH)* (H,NC)+ (HNC)’ (HCN)’ (H,O)+ (NH,)+ (NH,)’
28 28 27 27 18 17 16
11.67 12.96 11.52 11.74 13.13 10.57 13.34
tials of the fragmentations: only for m/z 44 has an experimental value been reported [II] : 10.24 eV, in excellent agreement with the values calculated for the isomers (H,NCO)’ and (HNCOH)‘. REFERENCES 1 J. R. Bews, C. Glidewell and P. H. Vidaud, J. Mol. Struct., 64 (1980) 75. (Part I). 2 J. A. Gilpin, Anal. Chem., 31 (1959) 4404. 3 D. W. Turner, C. Baker, A. D. Baker and C. R. Brundle, Molecular Photoelectron Spectroscopy, Wiley-Interscience, London, 1970. 4 J. R. Bews and C. Glidewell, 67 (1980) 151. (Part III). 5 R. C. Bingham, M. J. S. Dewar and D. H. Lo, J. Am. Chem. Sot., 97 (1975) 1285; QCPE, 10 (1976) 309. 6 G. Herzberg, Spectra of Diatomic Molecules, 2nd edn., Van Nostrand, Princeton, NJ, 1950. 7 B. N. Taylor, W. H. Parker and D. N. Langenberg, Rev. Mod. Phys., 41(1969) 477. 8 R. J. Kurland and E. B. Wilson, Jr., J. Chem. Phys., 27 (1957) 585. 9 C. Costain and J. M. Dowling, J. Chem. Phys., 32 (1960) 158. 10 H. Basch, M. B. Robin and N. A. Kuebler, J. Chem. Phys., 49 (1968) 5007. 11 H. M. Rosenstock, K. Draxl, B. W. Steiner and J. T. Herron, Energetics of Gaseous Ions, National Bureau of Standards, Washington, DC, 1977.