Does the Carbon-Sulfur triple bond exist? There’s probably little doubt it does in the CS molecule. But now Schreiner and Mloston have offered up the H-C≡S-OH species as a possibility.¹ Obtained by flash photolysis of 1, giving 2, and upon irradiation at 254 nm, H-C≡S-OH 3 is the observed species and not the expected carbene HO-C-SH 4. 3 is confirmed by excellent agreement between the observed and computationally predicted IR spectra.

The CCSD(T)/cc-pVTZ structures of 3 and 4 are shown in Figure 1. It is interesting that the carbene is not observed, even though it is 26.6 kcal mol^-1 more stable than 3.

3

4

Figure 1. CCSD(T)/cc-PVTZ optimized structures of 3 and 4.¹

So is there a triple bond? The short C-S distance (1.547 Å) is very similar to that in CS (1.545 Å). NBO analysis indicates a triple bond. But the MOs indicate significant lone pair build-up on both C and S, consistent with the strongly non-linear angles about these two atoms. The authors conclude that 3 is a “structure with a rather strong CS double bond or a weak triple bond”.

References

(1) Schreiner, P. R.; Reisenauer, H. P.; Romanski, J.; Mloston, G., "A Formal Carbon-Sulfur Triple Bond: H-C≡S-O-H," Angew. Chem. Int. Ed., 2009, 48, 8133-8136, DOI: 10.1002/anie.200903969

A computational study of addition of singlet carbenes to bicyclobutanes reveals another potential energy surface where dynamics may be active. Rablen, Jones and co-workers examined the reaction of dichlorocarbene with bicyclobutane 1 and 1,2,2-trimethylbicyclobutane 2 (Reactions 1 and 2) using a number of computational techniques.¹

For reaction 1, they identified three reaction pathways. The first two involve the carbene approaching along the central C-C bond. Path A (Scheme 1) involves a single transition state that leads to product 3, with a barrier of 8.4 kcal mol^-1. The second pat (pathway B), leads to critical point 4, which is a transition state at HF/6-31G* and QCISD/6-31G* but is a local minimum at CCSD/6-31G*. This minimum however is very shallow, and vibrational energy will exceed the barriers about it. Both pathways indicate an asynchronous but concerted reaction. The last pathway (C) is for insertion of the carbine into the bridgehead C-C bond, leading to the bicyclo product 5. This barrier is very high, 27 kcal mol^-1, and so this path is unlikely to be competitive.

Experimental study of Reaction 2 showed that only 6 is produced.² Rablen and Jones identified six pathways where the carbene attacks 2 along the bridgehead bond (analogous to Paths A and B, except there are three rotamers and the attack can be at either bridgehead carbon) and the insertion path that leads to 8. Once again, this last pathway has a very large barrier and is non-competitive. Attack at the unsubstituted bridgehead carbons is favored over attack at the methyl-substituted bridgehead by 2-3 kcal mol^-1. The path that leads directly to 7 has a slightly lower barrier (0.4 kcal mol^-1) than the path that leads directly to 8. The analog of Path B leads here to a true intermediate 9 through a barrier 0.4 kcal mol^-1 higher than the barrier that leads to 7. This intermediate is shown in Figure 1.

Figure 1. CCSD/6-31G* structure of intermediate 9.¹

The energies of the barriers suggest that 7 will be the major product, but not the exclusive product. Rablen and Jones point out that intermediate 9 lies in a very shallow plateau and exit from this intermediate can lead to either 7 or 8. This sort of potential energy surface has been implicated in reactions that exhibit non-statistical behavior indicative of dynamic effects (see Chapter 7 of my book). Rablen and Jones speculate that dynamics might be dictating the product distribution in Reaction 2 as well. Confirmation awaits a molecular dynamics study.

References

(1) Rablen, P. R.; Paiz, A. A.; Thuronyi, B. W.; Jones, M., "Computational Investigation of the Mechanism of Addition of Singlet Carbenes to Bicyclobutanes," J. Org. Chem. 2009, DOI: 10.1021/jo900485z

(2) Jackson, J. E.; Mock, G. B.; Tetef, M. L.; Zheng, G.-x.; Jones, M., "Reactions of carbenes with bicyclobutanes and quadricyclane : Cycloadditions with two σ bonds," Tetrahedron 1985, 41, 1453-1464, DOI: 10.1016/S0040-4020(01)96386-0.

InChIs

1: InChI=1/C4H6/c1-3-2-4(1)3/h3-4H,1-2H2
InChIKey=LASLVGACQUUOEB-UHFFFAOYAV

2: InChI=1/C7H12/c1-6(2)5-4-7(5,6)3/h5H,4H2,1-3H3
InChIKey=GJMVYBBYZUWWLJ-UHFFFAOYAI

3: InChI=1/C5H6Cl2/c1-2-3-4-5(6)7/h2,4H,1,3H2
InChIKey=FGUOQAVVVDPABB-UHFFFAOYAR

5: InChI=1/C5H6Cl2/c6-5(7)3-1-4(5)2-3/h3-4H,1-2H2
InChIKey=SUZACPSWEYRCBD-UHFFFAOYAW

6: InChI=1/C8H12Cl2/c1-6(2)8(3,4)5-7(9)10/h5H,1H2,2-4H3
InChIKey=QIFFCMZJZYIIBA-UHFFFAOYAZ

7: InChI=1/C8H12Cl2/c1-6(2)7(3)4-5-8(9)10/h5H,4H2,1-3H3
InChIKey=MOELQSRRCNAPQV-UHFFFAOYAX

8: InChI=1/C8H12Cl2/c1-6(2)5-4-7(6,3)8(5,9)10/h5H,4H2,1-3H3
InChIKey=PRCOWYZGJRWGOB-UHFFFAOYAP

Following on the great study of hydroxycarbene¹ (see my blog post), Schreiner now reports on the synthesis and characterization of dihydroxycarbene 1.² It is prepared by high-vacuum flash pyrolysis of oxalic acid (Scheme 1).

Scheme 1

Dihydroxycarbene can exist in three different conformations characterized by the relationship about the C-O bond, either s-cis or s-trans. The three conformations are shown in Figure 1, and the s-trans,s-trans structure is the local energy minimum (computed at CCSD(T)/cc-pVTZ).

1tt (0.0)

1ct (0.1)

1cc (6.7)

Figure 1. CCSD(T)/cc-pVTZ optimized geometries and relative energies (kcal mol^-1) of the conformers of 1.²

Identification of the 1 is made through comparison of the experimental and computed IR vibrational frequencies. As an example, the experimental and computed frequencies for the s-trans,s-trans conformer are listed in Table 1. The agreement is excellent.

Table 1. Computed and experimental vibrational frequencies (cm^-1) and intensities (in parentheses) of the s-trans,s-trans conformation of 1.²

Unlike hydroxycarbene, dihydroxycarbene is stable. The amazing instability of hydroxycarbene is due to tunneling through a large barrier: nearly 30 kcal mol^-1.¹ The tunneling route for the decomposition of 1 is more difficult for two reasons. First, its C-O bond is quite strong; the C-O distance is quite short, 1.325 Å. This makes a long distance that must be traversed in the tunneling mode. (The strong bond is due to π-donation from the oxygen lone pair into the empty carbon p orbital; this is noted by the large rotational barrier about the C-O bonds of 17 kcal mol^-1!) Second, the activation barrier for decomposition is very high, at least 34 kcal mol^-1.

References

(1) Schreiner, P. R.; Reisenauer, H. P.; Pickard Iv, F. C.; Simmonett, A. C.; Allen, W. D.; Matyus, E.; Csaszar, A. G., "Capture of hydroxymethylene and its fast disappearance through tunnelling," Nature, 2008, 453, 906-909, DOI: 10.1038/nature07010.

(2) Schreiner, P. R.; Reisenauer, H. P., "Spectroscopic Identification of Dihydroxycarbene13," Angew. Chem. Int. Ed., 2008, 47, 7071-7074, DOI: 10.1002/anie.200802105

InChIs

1: InChI=1/CH2O2/c2-1-3/h2-3H

A nice summary of the tunneling behavior of hydroxymethylene¹ was just published by Bucher in Angewandte Chemie.² Bucher strongly points out that the really novel part of this work is the very large barrier through which the proton tunnels. My blog post on this topic is here.

References

(1) Schreiner, P. R.; Reisenauer, H. P.; Pickard IV, F. C.; Simmonett, A. C.; Allen, W.
D.; Matyus, E.; Csaszar, A. G., "Capture of hydroxymethylene and its fast disappearance through tunnelling," Nature, 2008, 453, 906-909, DOI: 10.1038/nature07010.

(2) Bucher, G.; “Hydroxycarbene: Watching a Molecular Mole at Work,” Angew. Chem. Int. Ed., 2008, 47, 6957 – 6958, DOI: 10.1002/anie.200803195

The very simple carbene hydroxymethylene, HOCH, has finally been prepared and characterized.¹ Glyoxylic acid CHOCO₂H is subjected to high-vacuum laser photolysis. It fragments into HOCH, which is then trapped into an argon matrix. The experimental IR frequencies match up very well with the CCSD(T)/cc-pVQZ harmonic frequencies of the trans isomer 1t that are also adjusted for anharmonic effects. The computed vertical excitation energy of 415 nm matches well with the experimental value of the maximum absorption in the UV/vis spectra of 427 nm.

The other very interesting experimental result is that HOCH has a lifetime of about 2 hours in the matrix, while the deuterated species DOCH is stable. To explain these results, Schreiner, Allen and co-workers optimized a number of structures on the PES at CCSD(T)/cc-pVQZ and computed their energies using the focal point technique. The optimized structures and their relative energies are given in Figure 1.

1t (0.0)	TS2 (29.7)	2 (-52.1)
TS1(26.8)
1c (4.4)

Figure 1. Optimized CCSD(T)/cc-pVQZ structures of HOCH isomers and their Focal Point relative energies (kcal mol^-1).¹

The barriers for rearrangement from 1t are both very high. Rearrangement to formaldehyde 2 requires crossing a barrier of 29.7 kcal mol^-1, while the barrier to convert to the cis isomer 1c is 26.8 kcal mol^-1. (Note that from 1c a cleavage into CO and H2 can occur, but this barrier is another 47.0 kcal mol^-1.) These barriers are too large to be crossed at the very low temperatures of the matrices. However, using the intrinsic reaction potential at CCSD(T)/cc-pVQZ and WKB theory, the tunneling lifetime of HOCH is computed to be 122 minutes, in excellent accord with the experiment. The lifetime for DOCH is computed to be over 1200 years. Thus, the degradation of hydroxymethylene is entirely due to tunneling through a very large classical barrier! This rapid tunneling casts serious doubt on the ability to ever identify any hydroxymethylene in interstellar space.

References

(1) Schreiner, P. R.; Reisenauer, H. P.; Pickard IV, F. C.; Simmonett, A. C.; Allen, W.
D.; Matyus, E.; Csaszar, A. G., "Capture of hydroxymethylene and its fast disappearance through tunnelling," Nature, 2008, 453, 906-909, DOI: 10.1038/nature07010.

InChI

1: InChI=1/CH2O/c1-2/h1-2H
2: InChI=1/CH2O/c1-2/h1H2

In the book I extensively discuss the singlet-triplet gap of methylene and some of the chemistry of phenylcarbene. Schleyer and Schaefer have now reported computations on the singlet-triplet gap of arylcarbenes.¹ The geometries of phenylcarbene 1, diphenylcarbene 2, 1-naphthylcarbene 3, bis(1-naphtyl)carbene 4, and 9-anthrylcarbene 5 were optimized at B3LYP/6-311+G(d,p). These geometries are shown in Figure 1.

1s	1t
2s	2t
3s	3t
4s	4s
4s	4s

Figure 1. B3LYP/6-311+G(d,p) optimized structures of singlet and triplet 1-5.

Since this functional is known to underestimate the singlet-triplet gap of carbenes, they employ an empirical correction based on the difference in this gap for methylene between the computed value (11.89 kcal mol^-1) and the experimental value (9.05 kcal mol^-1). These corrected energy gaps are listed in Table 1.

Table 1. Corrected singlet-triplet energy gaps (kcal mol^-1) at B3LYP/6-311+G(d,p).

Using the following isodesmic reactions, they estimate the stabilization of the singlet or triplet carbene afforded by the aryl substituent:

R-C-H + CH₄ → H-C-H + R-CH₃

R-C-R + CH₄ → R-C-H + R-CH₃

These isodesmic energies are listed in Table 2. For phenylcarbne, the phenyl group stabilizes the singlet more than the triple, reducing the ST gap by 6.3 kcal mol^-1. However, adding a second phenyl group (making 2) stabilizes both the singlet and triplet by about the same amount, leading to little change in the ST gap. The singlet does not get accrue the potential benefit of the second aryl group because sterics prohibit the two rings from being coplanar.

Table 2. Aryl effect for 1-5 based on the isodesmic reaction energies (kcal mol^-1)

References

(1) Woodcock, H. L.; Moran, D.; Brooks, B. R.; Schleyer, P. v. R.; Schaefer, H. F., "Carbene Stabilization by Aryl Substituents. Is Bigger Better?," J. Am. Chem. Soc., 2007, 129, 3763-3770, DOI: 10.1021/ja068899t.

InChIs

1: InChI=1/C7H6/c1-7-5-3-2-4-6-7/h1-6H

2: InChI=1/C13H10/c1-3-7-12(8-4-1)11-13-9-5-2-6-10-13/h1-10H

3: InChI=1/C11H8/c1-9-5-4-7-10-6-2-3-8-11(9)10/h1-8H

4: InChI=1/C21H14/c1-2-8-19-14-16(12-13-17(19)6-1)15-20-10-5-9-18-7-3-4-11-21(18)20/h1-14H

5: InChI=1/C15H10/c1-11-14-8-4-2-6-12(14)10-13-7-3-5-9-15(11)13/h1-10H