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Interconversions between oxalic acid monoamide rotamers: Photochemical process versus tunneling
Journal of Molecular Structure 1025 (2012) 2–5
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Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Interconversions between oxalic acid monoamide rotamers: Photochemical process versus tunneling Günther Maier ⇑, Jörg Endres, Hans Peter Reisenauer ⇑ Institut für Organische Chemie der Justus-Liebig-Universität, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history: Available online 24 April 2012
UV irradiation converts matrix isolated (E,E)-oxalic acid monoamide (4) to its energetically less favorable (E,Z)-rotamer 5. The back-reaction 5 ? 4 seems to be governed by [H]tunneling. Matrix IR spectroscopy in combination with density functional computations is the method of choice to identify the bridged and unbridged rotamers 4 and 5. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Rotamers Energy levels Matrix isolation infrared spectroscopy Photochemistry Quantum tunneling DFT computations
1. Introduction Matrix isolation in combination with experimental and calculated IR spectra is an ideal technique for the synthesis and structural elucidation of carbenes [1]. For instance, in earlier studies we have shown that irradiation of thiazole-2-carboxylic acid 1a [2] and imidazole-2-carboxylic acid 1b [3] in an argon matrix at 10 K is the perfect way to isolate carbenes 2a and 2b. Both represent the parent compounds within the class of nucleophilic singlet carbenes of the Arduengo type [4]. Comparison of the experimental data with quantum-chemical calculations revealed their electronic peculiarities. H
H N C X
O C
10 K - CO 2
condensed phase
N
hν
C
N C
60 K
X
X
H
O 1a,b
combin. flash pyrolysis / matrix isolation a: X = S; b: X = NH
2a,b
The calculated activation barriers for the thermal isomerization of carbenes 2a and 2b into thiazole 3a and imidazole 3b are relatively high (42.3 and 41.5 kcal mol 1). Experimentally, 2a and 2b were transformed into 3a and 3b in condensed phase at about 60 K. Knowing this, it was no surprise that combination of flash pyrolysis of 1a and 1b with matrix isolation allowed only the detection of the stable aromatics 3a and 3b. Obviously carbenes 2a and 2b had been formed, but experienced a fast hydrogen shift (probably intermolecularly) under the applied reaction conditions. Our hope was, that decarboxylation of carboxylic acids of type 1 should also be suitable for the generation, matrix isolation and structural analysis of more simple carbenes. For instance, one of our goals was the preparation of aminohydroxycarbene 8, starting from oxalic acid monoamide 4.
3a,b
⇑ Corresponding authors. Fax: +49 (0)641 99 34309. E-mail addresses: [email protected] (G. Maier), [email protected] (H.P. Reisenauer). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.04.042
2. Computations For oxalic acid monoamide 4 four different rotamers can be envisioned (rotations around the bonds attached to the carboxylic carbonyl group). According to our calculations [B3LYP/6-311 + G(d,p); see SI for geometrical parameters, cartesian coordinates, and harmonic vibrational frequencies] the bridged species 4 is the most stable one [5]. Such a situation also exists in the case of oxalic acid [6]. This structure is preferred due to the interaction between the proton of the carboxyl and the oxygen atom of the amide group. The unbridged rotamer 5 is 3.4 kcal mol 1 higher in energy. Conformer 6 with cis-oriented carbonyl groups lies 5.2 kcal mol 1 above 4, the fourth candidate 7 does not represent a minimum on the energy hypersurface.
3
G. Maier et al. / Journal of Molecular Structure 1025 (2012) 2–5
H
O
H
C
O
O H
O
C
O
N
C
N
C
H
O
H
O
H H
O
C
O
N
C
H
O
H
H
(E,E)- 4 (0.0)
(E,Z)-5 (3.4)
C N
(Z,Z)- 6 (5.2)
The calculated transition state for the isomerisation 4 ? 5 by rotation around the CAO bond amounts to 13.7 kcal mol 1, resulting in an activation barrier of 10.3 kcal mol 1 for the thermal backreaction 5 ? 4. So the expectation was that both conformers should easily be detectable under matrix conditions.
3. Results and discussion 3.1. Irradiation of oxalic acid monoamide 4
O C
H
O H
(Z,E)- 7 ( - )
of 5 (Fig. 2, Table 1). After an irradiation time of 3 h a photo-equilibrium between 4 and 5 is reached.
Table 1 Comparison of experimental and calculated [B3LYP/6-311 + G(d,p), unscaled] wavenumbers of oxalic acid monoamide stereoisomers 4 and 5. Corresponding data for the trideuterated derivatives D3-4 and D3-5 are also included (prefix D:).
In agreement with the calculations experiment shows that only isomer 4 is present when oxalic acid monoamide is isolated in argon at 10 K (Fig. 1, Table 1). The strongest bands are situated at 1737.6 (m C@O, amide) and 1302.7 cm 1 (d HOC, acid). Upon irradiation of 4 with light of the wavelength k = 254 nm a strong new band at 1751.1 cm 1 together with a set of additional absorptions appears. They all belong to stereoisomer 5, as derived from the experimental difference spectrum and comparison with the calculated spectrum
m~exp 4, D3-4/
m~calc 4, D3-4/cm
cm 1 (Irel)
(Irel)a
3536.2 (mas NH2, m) D: –c 3417.4 (m OH, m) D: –c 3370.8 (ms NH2, m) D: –c 1815.9/1799.9 (m) D: 1799.1 (m) 1737.6 (s) D: 1728.5 (s) 1569.2 (m) D: – 1421.8 (m) D: 1266.1 (m) 1302.7 (s) D: – 1183.0 (w) D: 969.3 (w) – – 795.5 (w) D: – 705.1 (m) D: – – 628.2 (w) D: 522.3 (m) – 436.5 (m) D: – – 405.8 (m) D: – – – a
m~exp 5, D3-5/ cm 1 (Irel)
(Irel)b
3555.0 (m OH, m) D: –c 3545.5 (mas NH2, m) D: –c 3427.7 (ms NH2, w) D: –c 1769.4 (vw)
3753.2 (m OH, 0.13) D: 2757.8 (0.08) 3719.3 (mas NH2, 0.13) D: 2730.5 (0.09) 3585.1 (ms NH2, 0.09) D: 2587.9 (0.09) 1803.5 (0.04)
D: – 1751.1 (vs)
D: 1794.1 (0.11) 1791.3 (1.00)
1746.2 (vs) 1566.2 (m) D: – – D: 1274.0 (m) –
D: 1787.4 (1.00) 1601.2 (0.17) D: 1407.3 (0.03) 1403.6 (0.02) D: 1287.1 (0.55) 1309.2 (0.10) D: 1118.1 (0.04) 1181.6 (0.46) D: 1021.1 (0.09) 1093.9 (0.01) D: 933.9 (0.01) 832.6 (0.01) D: 829.5 (0.00) 772.5 (0.01) 684.3 (0.02) 661.8 (0.18) D: 565.4 (0.14) 617.6 (0.05) D: 556.3 (0.11) 614.8 (0.11) D: 508.9 (0.01) 526.4 (0.00) D: 448.0 (0.03) 433.4 (0.02) D: 386.7 (0.00) 413.5 (0.01) D: 365.1 (0.00) 349.2 (0.32) D: 263.9 (0.20) 269.2 (0.02) D: 254.8 (0.02) 61.8 (0.01) D: 60.9 (0.01)
1144.2 (m) D: 1001.8 (w) – – – 607.5 (m) D: 548.1 (w) – D: 544.4 (w) 590.1 (w) D: – – – – – – –
m~calc 5, D3-5/cm
1
Based on the band at 1335.4 cm 1 (abs. int. 386.4 km mol 1). Based on the band at 1791.3 cm 1 (abs. int. 669.9 km mol 1). c Due to the high acidity of the deuterons a back exchange by protons cannot be excluded during deposition. Therefore precise identification is not possible. b
Fig. 1. Comparison of the experimental IR spectrum (argon, 10 K) of oxalic acid monoamide with the calculated spectra (B3LYP/6-311 + G⁄⁄, unscaled) of rotamers 4, 5, and 6.
3709.0 (mas NH2, 0.24) D: 2750.2 (0.14) 3598.7 (m OH, 0.40) D: 2618.7 (0.22) 3576.5 (ms NH2, 0.17) D: 2582.2 (0.15) 1845.0 (m C@O, 0.72) D: 1836.6 (0.79) 1772.5 (m C@O, 0.98) D: 1763.1 (0.95) 1604.3 (0.19) D: 1430.5 (0.05) 1424.5 (0.42) D: 1281.8 (1.00) 1335.4 (1.00) D: 1130.9 (0.04) 1197.1 (0.05) D: 990.5 (0.08) 1100.2 (0.01) D: 939.5 (0.05) 822.2 (0.00) D: 817.9 (0.00) 798.2 (0.03) D: 759.3 (0.03) 730.0 (0.30) D: 582.3 (0.03) 651.7 (0.01) D: 534.4 (0.11) 632.4 (0.04) D: 529.3 (0.13) 542.1 (0.01) D: 518.8 (0.01) 471.5 (0.39) D: 408.0 (0.11) 402.4 (0.02) D: 373.4 (0.02) 398.1 (0.27) D: 306.7 (0.20) 268.6 (0.11) D: 249.9 (0.09) 112.2 (0.02) D: 109.7 (0.02)
1
4
G. Maier et al. / Journal of Molecular Structure 1025 (2012) 2–5
Fig. 2. Experimental difference spectrum (argon, 10 K) after irradiation of bridged oxalic acid monoamide 4 and comparison of the experimental IR spectrum of the photoproduct with the calculated spectrum (B3LYP/6-311 + G⁄⁄, unscaled) of the unbridged rotamer 5.
From visualization of the respective vibrational modes we know that the computed bands for 4 at 3598.7 cm 1 and for 5 at 3753.2 cm 1 have to be attributed to OAH stretching vibrations. There is a satisfying agreement between the calculated (154.5 cm 1) and experimental shift of 137.6 cm 1 (3417.4 versus 3555.0 cm 1) to higher wavenumbers, when going from the bridged to the unbridged stereoisomer. The same effect was observed in our earlier studies on the structural identification of matrix-isolated rotamers of acids 1a and 1b [2,3]. Even after long-time (20 h) irradiation of 4 with k = 254 nm only a partial decarboxylation could be achieved, but there was no hint for the formation of aminohydroxycarbene 8. When the low pressure mercury lamp was replaced by a KrF excimer laser (k = 248 nm), after 5 h complete decarboxylation of 4 was reached. But again, no signals were observed, which could be attributed to the calculated IR frequencies of 8. One of the final photo products was HNCO. There seems to exist a second fragmentation channel. Two bands at 1059.1 and 998.5 cm 1 indicated that in addition a complex between the fragments CO2, CO and NH3 had been formed. Presumably the wanted carbene 8 is formed in a first photochemical step, but is split further during irradiation via two subsequent reaction routes [9]. A similar photo-behavior is observed for dihydroxycarbene [10].
of 10.3 km mol 1 for the back-formation of 4 from 5, this reaction was found to occur even under matrix conditions (argon, 10 K) in the absence of any excitation by UV light. This process could be accelerated by IR radiation from the glowbar of the spectrometer and proceeded during the measurement of the IR spectrum of 5. Its half-life upon irradiation within the IR region (400–4000 cm 1) of the electromagnetic spectrum was roughly 90 min. These findings fit nicely to a study of Nieminen et al. [6]. They not only found in similar experiments with matrix-isolated oxalic acid, that the more stable conformer can be transformed into the less stable one by irradiation with 254 nm-light, and the back-reaction needs ~ > 2000 cm 1 ). only activation by IR irradiation (‘‘IR rotamerization’’; m Second, to our big surprise the back-reaction 5 ? 4 in argon at 10 K occurred even in complete darkness. For instance, if the matrix containing a mixture of 4 and 5 (prepared by irradiation of 4 with 248- or 254-nm light) stands in the dark at 10 K, one registers a decrease of the bands of 5 and the absorptions of 4 increase. If it is taken into account that 5 can only partially be transferred into 4 by IR excitation while measuring the spectrum, there remains a big part of 5, which has to form 4 via a spontaneous process. That means, this process needs no photoactivation at all. The measured half-life for molecules following such a pathway can be estimated to lie around 450 min. If one trusts in the calculated barrier for the thermal backreaction 5 ? 4 it is tempting to assume, that quantum tunneling dominates this process. A reasonable test should be the exchange of the proton against a deuteron. For this reason all three protons in 4 were replaced by deuterons. If the same matrix experiments described above were repeated with perdeuterated D3-4, the results did not change (Table 1), with one exception: Contrary to 5 isotopomer D3-5 was thermally stable in argon at 10 K. The matrix IR spectrum of the mixture of D3-4 and D3-5 remained completely unchanged upon standing in the dark, no decrease of the bands of D3-5 could be observed. This is a strong argument that tunneling plays an important role in case of the protonated species 5. At the time, when we carried our experiments [7], similar results were reported by Räsänen et al. [8]. This group describes the photochemical isomerisation of the matrix-isolated bridged form of formic acid into the unbridged conformer and the reverse process via ‘‘tunneling through the torsional barrier’’. In the meantime additional experimental and computational studies have shown that tunneling processes of the type discussed above are a common feature of carboxylic acids [11]. 4. Conclusions Whereas matrix photolyses of thiazole-2-carboxylic acid 1a and imidazole-2-carboxylic acid 1b are perfect precursors for the preparation of carbenes 2a,b, the same procedure is not applicable for the matrix isolation of aminohydroxycarbene 8 by photochemical decarboxylation of oxalic acid monoamide 4. Upon irradiation 4 isomerizes to the energetically less favorable unbridged rotamer 5. The back-reaction can not only be induced by IR irradiation, but proceeds even without external excitation at 10 K in the dark. By deuteration this process can be totally inhibited, indicating a [H]tunneling reaction mechanism. Prolonged matrix photolysis of 4, especially with a KrF excimer laser, leads to complete splitting into HNCO, CO, CO2, and NH3. 5. Experimental section
3.2. Spontaneous rotamerization of oxalic acid monoamide 5
5.1. Oxalic acid monoamide 4
The disappointing photo-behavior of 4 was well-balanced by two surprising observations. First, in spite of a calculated barrier
Was commercially available (Lancaster). Deuteration of 4 was achieved by recrystallization of 4 in D2O.
G. Maier et al. / Journal of Molecular Structure 1025 (2012) 2–5
5
5.2. Matrix isolation experiments
References
The cryostat used for the matrix isolation studies was an APD Cryogenics HC-2 closed-cycle refrigerator system fitted with CsI windows for IR and BaF2 windows for UV/Vis measurements. Matrix temperature was measured and controlled by a Scientific Instruments 9600-1 silicon diode temperature controller. For irradiations a mercury low-pressure spiral lamp (Gräntzel) with a vycor filter and a KrF excimer laser LPX 100 (Lambda Physik) was used. IR spectra were recorded with a Bruker IFS 55 FTIR spectrometer (4500–300 cm 1, resolution 0.7 cm 1), while UV/Vis spectra were measured with diode array spectrophotometer HP8453 (Agilent). For matrix isolation 4 was sublimed at 65 °C and deposited on a CsI window at 10 K together with a large excess of argon (molar ratio ca. 1:1000).
[1] G. Maier, H.P. Reisenauer, in: U.H. Brinker (Ed.), Advances in Carbene Chemistry, vol. 3, Elsevier, Amsterdam, 2001, pp. 115–157. [2] G. Maier, J. Endres, H.P. Reisenauer, Angew. Chem. 109 (1997) 1788–1790; G. Maier, J. Endres, H.P. Reisenauer, Angew. Chem. Int. Ed. Engl. 36 (1997) 1709–1712. [3] G. Maier, J. Endres, Eur. J. Org. Chem. (1998) 1517–1520. [4] Review: W.A. Herrmann, C. Köcher, Angew. Chem. 109 (1997) 2257–2282; W.A. Herrmann, C. Köcher, Angew. Chem. Int. Ed. Engl. 36 (1997) 2162–2187. [5] All calculations were carried out by using the program package Gaussian 94. Zero point energies are included in calculated energies. M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. Head-Gordon, C. Gonzalez, J.A. Pople, Gaussian 94, Revision D.3, Gaussian, Inc., Pittsburgh PA, 1995. [6] J. Nieminen, M. Räsänen, J. Murto, J. Phys. Chem. 96 (1992) 5303–5308. [7] J. Endres, Dissertation, University of Giessen, 1999. [8] M. Pettersson, J. Lundell, L. Khriachtchev, M. Räsänen, J. Am. Chem. Soc. 119 (1997) 11715–11716; see also:M. Pettersson, E.M.S. Maçôas, L. Khriachtchev, J. Lundell, R. Fausto, M. Räsänen, J. Chem. Phys. 117 (2002) 9095–9098. [9] Matrix isolated aminohydroxycarbene 8 (accessible by flash vacuum pyrolysis of 4) exhibits a UV absorption at kmax = 248 nm. (H.P. Reisenauer, unpublished result). [10] P.R. Schreiner, H.P. Reisenauer, Angew. Chem. Int. Ed. 47 (2008) 7071–7074. [11] S. Amiri, H.P. Reisenauer, P.R. Schreiner, J. Am. Chem. Soc. 132 (2010) 15902– 15904.
Acknowledgments This work was supported by the Fonds der Chemischen Industrie and the Deutsche Forschungsgemeinschaft. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molstruc.2012.04. 042.
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Interconversions between oxalic acid monoamide rotamers: Photochemical process versus tunneling Günther Maier ⇑, Jörg Endres, Hans Peter Reisenauer ⇑ Institut für Organische Chemie der Justus-Liebig-Universität, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history: Available online 24 April 2012
UV irradiation converts matrix isolated (E,E)-oxalic acid monoamide (4) to its energetically less favorable (E,Z)-rotamer 5. The back-reaction 5 ? 4 seems to be governed by [H]tunneling. Matrix IR spectroscopy in combination with density functional computations is the method of choice to identify the bridged and unbridged rotamers 4 and 5. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Rotamers Energy levels Matrix isolation infrared spectroscopy Photochemistry Quantum tunneling DFT computations
1. Introduction Matrix isolation in combination with experimental and calculated IR spectra is an ideal technique for the synthesis and structural elucidation of carbenes [1]. For instance, in earlier studies we have shown that irradiation of thiazole-2-carboxylic acid 1a [2] and imidazole-2-carboxylic acid 1b [3] in an argon matrix at 10 K is the perfect way to isolate carbenes 2a and 2b. Both represent the parent compounds within the class of nucleophilic singlet carbenes of the Arduengo type [4]. Comparison of the experimental data with quantum-chemical calculations revealed their electronic peculiarities. H
H N C X
O C
10 K - CO 2
condensed phase
N
hν
C
N C
60 K
X
X
H
O 1a,b
combin. flash pyrolysis / matrix isolation a: X = S; b: X = NH
2a,b
The calculated activation barriers for the thermal isomerization of carbenes 2a and 2b into thiazole 3a and imidazole 3b are relatively high (42.3 and 41.5 kcal mol 1). Experimentally, 2a and 2b were transformed into 3a and 3b in condensed phase at about 60 K. Knowing this, it was no surprise that combination of flash pyrolysis of 1a and 1b with matrix isolation allowed only the detection of the stable aromatics 3a and 3b. Obviously carbenes 2a and 2b had been formed, but experienced a fast hydrogen shift (probably intermolecularly) under the applied reaction conditions. Our hope was, that decarboxylation of carboxylic acids of type 1 should also be suitable for the generation, matrix isolation and structural analysis of more simple carbenes. For instance, one of our goals was the preparation of aminohydroxycarbene 8, starting from oxalic acid monoamide 4.
3a,b
⇑ Corresponding authors. Fax: +49 (0)641 99 34309. E-mail addresses: [email protected] (G. Maier), [email protected] (H.P. Reisenauer). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.04.042
2. Computations For oxalic acid monoamide 4 four different rotamers can be envisioned (rotations around the bonds attached to the carboxylic carbonyl group). According to our calculations [B3LYP/6-311 + G(d,p); see SI for geometrical parameters, cartesian coordinates, and harmonic vibrational frequencies] the bridged species 4 is the most stable one [5]. Such a situation also exists in the case of oxalic acid [6]. This structure is preferred due to the interaction between the proton of the carboxyl and the oxygen atom of the amide group. The unbridged rotamer 5 is 3.4 kcal mol 1 higher in energy. Conformer 6 with cis-oriented carbonyl groups lies 5.2 kcal mol 1 above 4, the fourth candidate 7 does not represent a minimum on the energy hypersurface.
3
G. Maier et al. / Journal of Molecular Structure 1025 (2012) 2–5
H
O
H
C
O
O H
O
C
O
N
C
N
C
H
O
H
O
H H
O
C
O
N
C
H
O
H
H
(E,E)- 4 (0.0)
(E,Z)-5 (3.4)
C N
(Z,Z)- 6 (5.2)
The calculated transition state for the isomerisation 4 ? 5 by rotation around the CAO bond amounts to 13.7 kcal mol 1, resulting in an activation barrier of 10.3 kcal mol 1 for the thermal backreaction 5 ? 4. So the expectation was that both conformers should easily be detectable under matrix conditions.
3. Results and discussion 3.1. Irradiation of oxalic acid monoamide 4
O C
H
O H
(Z,E)- 7 ( - )
of 5 (Fig. 2, Table 1). After an irradiation time of 3 h a photo-equilibrium between 4 and 5 is reached.
Table 1 Comparison of experimental and calculated [B3LYP/6-311 + G(d,p), unscaled] wavenumbers of oxalic acid monoamide stereoisomers 4 and 5. Corresponding data for the trideuterated derivatives D3-4 and D3-5 are also included (prefix D:).
In agreement with the calculations experiment shows that only isomer 4 is present when oxalic acid monoamide is isolated in argon at 10 K (Fig. 1, Table 1). The strongest bands are situated at 1737.6 (m C@O, amide) and 1302.7 cm 1 (d HOC, acid). Upon irradiation of 4 with light of the wavelength k = 254 nm a strong new band at 1751.1 cm 1 together with a set of additional absorptions appears. They all belong to stereoisomer 5, as derived from the experimental difference spectrum and comparison with the calculated spectrum
m~exp 4, D3-4/
m~calc 4, D3-4/cm
cm 1 (Irel)
(Irel)a
3536.2 (mas NH2, m) D: –c 3417.4 (m OH, m) D: –c 3370.8 (ms NH2, m) D: –c 1815.9/1799.9 (m) D: 1799.1 (m) 1737.6 (s) D: 1728.5 (s) 1569.2 (m) D: – 1421.8 (m) D: 1266.1 (m) 1302.7 (s) D: – 1183.0 (w) D: 969.3 (w) – – 795.5 (w) D: – 705.1 (m) D: – – 628.2 (w) D: 522.3 (m) – 436.5 (m) D: – – 405.8 (m) D: – – – a
m~exp 5, D3-5/ cm 1 (Irel)
(Irel)b
3555.0 (m OH, m) D: –c 3545.5 (mas NH2, m) D: –c 3427.7 (ms NH2, w) D: –c 1769.4 (vw)
3753.2 (m OH, 0.13) D: 2757.8 (0.08) 3719.3 (mas NH2, 0.13) D: 2730.5 (0.09) 3585.1 (ms NH2, 0.09) D: 2587.9 (0.09) 1803.5 (0.04)
D: – 1751.1 (vs)
D: 1794.1 (0.11) 1791.3 (1.00)
1746.2 (vs) 1566.2 (m) D: – – D: 1274.0 (m) –
D: 1787.4 (1.00) 1601.2 (0.17) D: 1407.3 (0.03) 1403.6 (0.02) D: 1287.1 (0.55) 1309.2 (0.10) D: 1118.1 (0.04) 1181.6 (0.46) D: 1021.1 (0.09) 1093.9 (0.01) D: 933.9 (0.01) 832.6 (0.01) D: 829.5 (0.00) 772.5 (0.01) 684.3 (0.02) 661.8 (0.18) D: 565.4 (0.14) 617.6 (0.05) D: 556.3 (0.11) 614.8 (0.11) D: 508.9 (0.01) 526.4 (0.00) D: 448.0 (0.03) 433.4 (0.02) D: 386.7 (0.00) 413.5 (0.01) D: 365.1 (0.00) 349.2 (0.32) D: 263.9 (0.20) 269.2 (0.02) D: 254.8 (0.02) 61.8 (0.01) D: 60.9 (0.01)
1144.2 (m) D: 1001.8 (w) – – – 607.5 (m) D: 548.1 (w) – D: 544.4 (w) 590.1 (w) D: – – – – – – –
m~calc 5, D3-5/cm
1
Based on the band at 1335.4 cm 1 (abs. int. 386.4 km mol 1). Based on the band at 1791.3 cm 1 (abs. int. 669.9 km mol 1). c Due to the high acidity of the deuterons a back exchange by protons cannot be excluded during deposition. Therefore precise identification is not possible. b
Fig. 1. Comparison of the experimental IR spectrum (argon, 10 K) of oxalic acid monoamide with the calculated spectra (B3LYP/6-311 + G⁄⁄, unscaled) of rotamers 4, 5, and 6.
3709.0 (mas NH2, 0.24) D: 2750.2 (0.14) 3598.7 (m OH, 0.40) D: 2618.7 (0.22) 3576.5 (ms NH2, 0.17) D: 2582.2 (0.15) 1845.0 (m C@O, 0.72) D: 1836.6 (0.79) 1772.5 (m C@O, 0.98) D: 1763.1 (0.95) 1604.3 (0.19) D: 1430.5 (0.05) 1424.5 (0.42) D: 1281.8 (1.00) 1335.4 (1.00) D: 1130.9 (0.04) 1197.1 (0.05) D: 990.5 (0.08) 1100.2 (0.01) D: 939.5 (0.05) 822.2 (0.00) D: 817.9 (0.00) 798.2 (0.03) D: 759.3 (0.03) 730.0 (0.30) D: 582.3 (0.03) 651.7 (0.01) D: 534.4 (0.11) 632.4 (0.04) D: 529.3 (0.13) 542.1 (0.01) D: 518.8 (0.01) 471.5 (0.39) D: 408.0 (0.11) 402.4 (0.02) D: 373.4 (0.02) 398.1 (0.27) D: 306.7 (0.20) 268.6 (0.11) D: 249.9 (0.09) 112.2 (0.02) D: 109.7 (0.02)
1
4
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Fig. 2. Experimental difference spectrum (argon, 10 K) after irradiation of bridged oxalic acid monoamide 4 and comparison of the experimental IR spectrum of the photoproduct with the calculated spectrum (B3LYP/6-311 + G⁄⁄, unscaled) of the unbridged rotamer 5.
From visualization of the respective vibrational modes we know that the computed bands for 4 at 3598.7 cm 1 and for 5 at 3753.2 cm 1 have to be attributed to OAH stretching vibrations. There is a satisfying agreement between the calculated (154.5 cm 1) and experimental shift of 137.6 cm 1 (3417.4 versus 3555.0 cm 1) to higher wavenumbers, when going from the bridged to the unbridged stereoisomer. The same effect was observed in our earlier studies on the structural identification of matrix-isolated rotamers of acids 1a and 1b [2,3]. Even after long-time (20 h) irradiation of 4 with k = 254 nm only a partial decarboxylation could be achieved, but there was no hint for the formation of aminohydroxycarbene 8. When the low pressure mercury lamp was replaced by a KrF excimer laser (k = 248 nm), after 5 h complete decarboxylation of 4 was reached. But again, no signals were observed, which could be attributed to the calculated IR frequencies of 8. One of the final photo products was HNCO. There seems to exist a second fragmentation channel. Two bands at 1059.1 and 998.5 cm 1 indicated that in addition a complex between the fragments CO2, CO and NH3 had been formed. Presumably the wanted carbene 8 is formed in a first photochemical step, but is split further during irradiation via two subsequent reaction routes [9]. A similar photo-behavior is observed for dihydroxycarbene [10].
of 10.3 km mol 1 for the back-formation of 4 from 5, this reaction was found to occur even under matrix conditions (argon, 10 K) in the absence of any excitation by UV light. This process could be accelerated by IR radiation from the glowbar of the spectrometer and proceeded during the measurement of the IR spectrum of 5. Its half-life upon irradiation within the IR region (400–4000 cm 1) of the electromagnetic spectrum was roughly 90 min. These findings fit nicely to a study of Nieminen et al. [6]. They not only found in similar experiments with matrix-isolated oxalic acid, that the more stable conformer can be transformed into the less stable one by irradiation with 254 nm-light, and the back-reaction needs ~ > 2000 cm 1 ). only activation by IR irradiation (‘‘IR rotamerization’’; m Second, to our big surprise the back-reaction 5 ? 4 in argon at 10 K occurred even in complete darkness. For instance, if the matrix containing a mixture of 4 and 5 (prepared by irradiation of 4 with 248- or 254-nm light) stands in the dark at 10 K, one registers a decrease of the bands of 5 and the absorptions of 4 increase. If it is taken into account that 5 can only partially be transferred into 4 by IR excitation while measuring the spectrum, there remains a big part of 5, which has to form 4 via a spontaneous process. That means, this process needs no photoactivation at all. The measured half-life for molecules following such a pathway can be estimated to lie around 450 min. If one trusts in the calculated barrier for the thermal backreaction 5 ? 4 it is tempting to assume, that quantum tunneling dominates this process. A reasonable test should be the exchange of the proton against a deuteron. For this reason all three protons in 4 were replaced by deuterons. If the same matrix experiments described above were repeated with perdeuterated D3-4, the results did not change (Table 1), with one exception: Contrary to 5 isotopomer D3-5 was thermally stable in argon at 10 K. The matrix IR spectrum of the mixture of D3-4 and D3-5 remained completely unchanged upon standing in the dark, no decrease of the bands of D3-5 could be observed. This is a strong argument that tunneling plays an important role in case of the protonated species 5. At the time, when we carried our experiments [7], similar results were reported by Räsänen et al. [8]. This group describes the photochemical isomerisation of the matrix-isolated bridged form of formic acid into the unbridged conformer and the reverse process via ‘‘tunneling through the torsional barrier’’. In the meantime additional experimental and computational studies have shown that tunneling processes of the type discussed above are a common feature of carboxylic acids [11]. 4. Conclusions Whereas matrix photolyses of thiazole-2-carboxylic acid 1a and imidazole-2-carboxylic acid 1b are perfect precursors for the preparation of carbenes 2a,b, the same procedure is not applicable for the matrix isolation of aminohydroxycarbene 8 by photochemical decarboxylation of oxalic acid monoamide 4. Upon irradiation 4 isomerizes to the energetically less favorable unbridged rotamer 5. The back-reaction can not only be induced by IR irradiation, but proceeds even without external excitation at 10 K in the dark. By deuteration this process can be totally inhibited, indicating a [H]tunneling reaction mechanism. Prolonged matrix photolysis of 4, especially with a KrF excimer laser, leads to complete splitting into HNCO, CO, CO2, and NH3. 5. Experimental section
3.2. Spontaneous rotamerization of oxalic acid monoamide 5
5.1. Oxalic acid monoamide 4
The disappointing photo-behavior of 4 was well-balanced by two surprising observations. First, in spite of a calculated barrier
Was commercially available (Lancaster). Deuteration of 4 was achieved by recrystallization of 4 in D2O.
G. Maier et al. / Journal of Molecular Structure 1025 (2012) 2–5
5
5.2. Matrix isolation experiments
References
The cryostat used for the matrix isolation studies was an APD Cryogenics HC-2 closed-cycle refrigerator system fitted with CsI windows for IR and BaF2 windows for UV/Vis measurements. Matrix temperature was measured and controlled by a Scientific Instruments 9600-1 silicon diode temperature controller. For irradiations a mercury low-pressure spiral lamp (Gräntzel) with a vycor filter and a KrF excimer laser LPX 100 (Lambda Physik) was used. IR spectra were recorded with a Bruker IFS 55 FTIR spectrometer (4500–300 cm 1, resolution 0.7 cm 1), while UV/Vis spectra were measured with diode array spectrophotometer HP8453 (Agilent). For matrix isolation 4 was sublimed at 65 °C and deposited on a CsI window at 10 K together with a large excess of argon (molar ratio ca. 1:1000).
[1] G. Maier, H.P. Reisenauer, in: U.H. Brinker (Ed.), Advances in Carbene Chemistry, vol. 3, Elsevier, Amsterdam, 2001, pp. 115–157. [2] G. Maier, J. Endres, H.P. Reisenauer, Angew. Chem. 109 (1997) 1788–1790; G. Maier, J. Endres, H.P. Reisenauer, Angew. Chem. Int. Ed. Engl. 36 (1997) 1709–1712. [3] G. Maier, J. Endres, Eur. J. Org. Chem. (1998) 1517–1520. [4] Review: W.A. Herrmann, C. Köcher, Angew. Chem. 109 (1997) 2257–2282; W.A. Herrmann, C. Köcher, Angew. Chem. Int. Ed. Engl. 36 (1997) 2162–2187. [5] All calculations were carried out by using the program package Gaussian 94. Zero point energies are included in calculated energies. M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. Head-Gordon, C. Gonzalez, J.A. Pople, Gaussian 94, Revision D.3, Gaussian, Inc., Pittsburgh PA, 1995. [6] J. Nieminen, M. Räsänen, J. Murto, J. Phys. Chem. 96 (1992) 5303–5308. [7] J. Endres, Dissertation, University of Giessen, 1999. [8] M. Pettersson, J. Lundell, L. Khriachtchev, M. Räsänen, J. Am. Chem. Soc. 119 (1997) 11715–11716; see also:M. Pettersson, E.M.S. Maçôas, L. Khriachtchev, J. Lundell, R. Fausto, M. Räsänen, J. Chem. Phys. 117 (2002) 9095–9098. [9] Matrix isolated aminohydroxycarbene 8 (accessible by flash vacuum pyrolysis of 4) exhibits a UV absorption at kmax = 248 nm. (H.P. Reisenauer, unpublished result). [10] P.R. Schreiner, H.P. Reisenauer, Angew. Chem. Int. Ed. 47 (2008) 7071–7074. [11] S. Amiri, H.P. Reisenauer, P.R. Schreiner, J. Am. Chem. Soc. 132 (2010) 15902– 15904.
Acknowledgments This work was supported by the Fonds der Chemischen Industrie and the Deutsche Forschungsgemeinschaft. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molstruc.2012.04. 042.