Iminopropadienones R–NCCCO and carbon suboxide, C3O2. Theoretical and experimental 13C NMR spectra

Journal of Molecular Structure (Theochem) 686 (2004) 31–36 www.elsevier.com/locate/theochem

Iminopropadienones R–NaCaCaCaO and carbon suboxide, C3O2. Theoretical and experimental 13C NMR spectra Rainer Kocha,*, Torsten Bruhna, Rakesh Naduvile Veedub, Curt Wentrupb a Institut fu¨r Reine und Angewandte Chemie, Carl von Ossietzky Universita¨t, P.O. Box 2503, D-26111 Oldenburg, Germany Department of Chemistry, School of Molecular and Microbial Sciences, The University of Queensland, Brisbane, Qld 4072, Australia

b

Received 1 July 2004; revised 27 July 2004; accepted 29 July 2004

Abstract The 13C NMR data of five iminopropadienones R–NaCaCaCaO as well as carbon suboxide, C3O2, have been examined theoretically and experimentally. The best theoretical results were obtained using the GIAO/B3LYP/6-31CG**//MP2/6-31G* level of theory, which reproduces the chemical shifts of the iminopropadienone substituents extremely well while underestimating those of the cumulenic carbons by 5–10 ppm. The computationally faster GIAO/HF/6-31CG**//B3LYP/6-31G* level is also adequate. q 2004 Elsevier B.V. All rights reserved. Keywords: Iminopropadienones; 13C NMR calculations; Theoretical chemical shifts

1. Introduction Iminopropadienones, RNaCaCaCaO, are a class of recently discovered compounds usually generated by flash vacuum thermolysis (FVT) of suitable precursors [1–9]. Simple alkyl derivatives (methyl [1], isopropyl [2], and tertbutyl [2]) are highly unstable compounds which can only be characterized by low-temperature spectroscopy, but sterically hindering substituents make some of these compounds stable enough to be isolated at room temperature and characterized by 13C NMR spectroscopy (neopentyl 1 [7], mesityl 2 [7], and o-tert-butylphenyl-NCCCO 3 [7]). Furthermore, we have prepared the pentafluorophenyl 4 and a-naphthyl-iminopropaqdienone 5 by FVT of the corresponding Meldrum’s acid derivatives (5-[dimethylamino(arylamino)methylene]-1,3-dioxane-4,6-diones) [8,9]. Compounds 4 and 5 are stable in solution at K50 8C and even survive several days at room temperature. Therefore, they could be analyzed in detail by using 13C NMR (DEPT) spectroscopy as well as infrared spectroscopy and chemical derivatization reactions. * Corresponding author. Tel.: C49-441-798-3653; fax: C49-441-7983329. E-mail address: [email protected] (R. Koch). 0166-1280/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2004.07.027

The IR spectra of iminopropadienones are dominated by a very intense and sometimes complex band centered around 2250 cmK1, and a much weaker band in the 2100–2200 cmK1 range. IR spectra of compounds 1–3 are reported in the Supporting Information of Ref. [7], and other examples are given in Refs. [3,5,8]. Compound 4 features bands at 2270 (s) and 2217 (w) cmK1, and compound 5 at 2235 (s) and 2124 (w) cmK1 (Ar matrix, 20 K). The 13C NMR spectra of 1–3 show highly characteristic carbon resonances at K4 to K11 (central cumulenic carbon, C2), 108–112 (CaN), and 130–133 ppm (CaO) [7] (the 1H and 13C NMR spectra of 1–3 are reproduced in the Supporting Information of Ref. [7]). Carbon suboxide, OaCaCaCaO, features analogous 13C NMR resonances. There are two sets of experimental data for C3O2 in the literature, with signals at K14.6 and 129.7 ppm [10] and at K14.6 and 127.7 ppm [11,12], respectively, which differ only slightly in the low field signal. This may be due to the different methods of measurement; the second set of data being for the solid state. We have used the first set of data in this study. Since 13C NMR spectroscopy is crucial for the correct identification of the iminopropadienones, it is timely to examine the ability of modern computational methods to predict these spectra. We have therefore embarked on such

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R. Koch et al. / Journal of Molecular Structure (Theochem) 686 (2004) 31–36

perturbation theory methods (second order Møller–Plesset (MP2) [16,17]) were performed with GAUSSIAN 98 [18]. Pople’s 6-31G* [19,20] and 6-311CG** [21,22,23] basis sets were chosen. In a first step, 1–6 were optimized at various levels of theory (see below and tables for details). After confirmation of all structures as minima by calculating harmonic frequencies, relative chemical shifts were determined with the GIAO (gauge including (or invariant) atomic orbital) method [24–26] as implemented in GAUSSIAN 98. The obtained shielding tensors were referenced against tetramethylsilane (TMS) to yield relative chemical shifts. Atoms-in-molecules (AIM) analyses [27,28] were performed with AIM2000 [29] based on the B3LYP/6-31C G**//MP2/6-31G* wavefunctions.

3. Results and discussion In order to evaluate both a reliable and a computationally efficient theoretical level, the five RNCCCO representatives and C3O2 have been investigated with several combinations of NMR single point calculation and geometries. The relative 13C chemical shifts of the five iminopropadienones 1–5 are given in Tables 1–5. There are two sections of these molecules, the cumulenic part and the substituents R which will be treated separately in this discussion. The calculation of relative chemical shifts of aryl substituents (2–5) shows no significant dependency of a particular method. Almost all combinations used herein reproduce the signals with a mean accuracy of about 2 ppm. The situation is somewhat different with the neopentyl group in 1: only the DFT-NMR calculations, based on either MP2 or B3LYP geometries and the MP2-NMR results can give the desired precision of 2 ppm. HF approaches have much higher average errors. The more demanding part of the molecules investigated herein is certainly the cumulated double bond system. Experimentally, the 13C NMR signals for the outermost carbon atom C 1 (CZO) are essentially the same (129–133 ppm). This is also true for the calculated shifts

Chart 1.

a study of the iminopropadienones 1–5 as well as carbon suboxide 6. The results are reported herein (Chart 1).

2. Computational details In the present study all calculations with Hartree–Fock (HF) [13], density functional theory (DFT, employing the hybrid functional B3LYP [14,15]) and many body

Table 1 Experimental and calculated 13C NMR chemical shifts d (ppm) of 1, relative to TMS

Experimenta HF/6-31G*//HF/6-31G* HF/6-31CG**//HF/6-31G* HF/6-31G*//B3LYP/6-31G* HF/6-31CG**//B3LYP/6-31G* B3LYP/6-31G*//B3LYP/6-31G* B3LYP/6-31CG**//B3LYP/6-31G* HF/6-31G*//MP2/6-31G* HF/6-31CG**//MP2/6-31G* B3LYP/6-31CG**//MP2/6-31G* MP2/6-31G*//MP2/6-31G*

C1

C2

C3

C4

C5

CMe

CMe

CMe

130.0 119.0 120.9 126.1 127.6 108.7 112.9 130.1 131.5 115.4 108.4

K11.0 K24.0 K22.1 K12.3 K10.6 K8.3 K3.3 K8.9 K7.8 K1.5 K0.4

108.5 98.9 100.0 115.3 116.8 94.5 98.8 118.2 119.8 99.5 93.9

57.5 50.5 49.1 50.3 51.2 56.4 58.7 51.1 52.1 59.7 60.3

33.9 26.7 28.1 26.2 27.2 33.3 35.7 24.4 28.8 33.4 31.4

27.1 25.2 25.5 25.1 25.4 27.2 27.0 25.4 25.7 27.2 28.4

27.1 24.2 24.1 24.5 24.4 26.6 27.2 25.0 24.9 27.6 28.0

27.1 24.2 24.1 24.5 24.4 26.6 27.2 25.0 24.9 27.7 28.0

11.2 9.6 4.0 3.7 12.7 11.5 4.0 5.3 11.0 15.6

The first bold number is the average error per carbon atom (ppm) for the cumulenic part, the second for the substituent R. a Ref. [7], solvent CD3OD.

4.4 4.4 4.4 4.0 0.9 1.1 4.4 3.3 0.8 1.7

R. Koch et al. / Journal of Molecular Structure (Theochem) 686 (2004) 31–36

33

Table 2 Experimental and calculated 13C NMR chemical shifts d (ppm) of 2, relative to TMS

a

Experiment HF/6-31G*//HF/6-31G* HF/6-31CG**//HF/6-31G* HF/6-31G*//B3LYP/6-31G* HF/6-31CG**//B3LYP/6-31G* HF/6-31G*//MP2/6-31G* HF/6-31CG**//MP2/6-31G* B3LYP/6-31CG**//MP2/6-31G*

C1

C2

C3

132.5 123.7 125.2 131.1 132.4 135.6 137.0 121.8

K6.8 K21.9 K20.1 K12.9 K11.3 K8.9 K7.8 2.8

111.6 102.5 104.8 114.7 117.2 119.1 121.9 105.2

11.0 9.1 3.5 3.4 4.2 5.3 8.9

C4

C5

C6

C7

C8

C9

CMe

CMe

CMe

126.2 122.6 123.7 126.0 126.9 126.7 127.6 124.0

135.0 135.5 137.9 139.5 141.9 139.9 142.2 133.2

128.8 125.9 125.2 127.1 126.5 127.6 127.1 125.3

137.3 137.0 140.4 137.4 140.6 137.6 140.8 134.2

128.8 125.6 125.3 126.9 126.3 127.4 126.9 125.6

135.0 135.5 137.8 135.7 137.9 135.5 137.7 137.8

18.6 19.4 18.8 19.6 19.1 20.0 19.5 21.4

21.1 20.4 20.2 20.4 20.2 20.7 20.5 22.7

18.6 19.3 18.7 19.5 18.9 19.6 19.1 21.5

1.5 2.2 1.3 2.3 1.3 2.3 2.7

The first bold number is the average error per carbon atom (ppm) for the cumulenic part, the second for the substituent R. a Ref. [7], solvent CDCl3.

Table 3 Experimental and calculated 13C NMR chemical shifts d (ppm) of 3, relative to TMS

Experimenta HF/6-31G*//HF/6-31G* HF/6-31CG**//HF/6-31G* HF/6-31G*//B3LYP/6-31G* HF/6-31CG**//B3LYP/6-31G* HF/6-31G*//MP2/6-31G* HF/6-31CG**//MP2/6-31G* B3LYP/6-31CG**//MP2/6-31G*

C1

C2

C3

C4

C5

C6

C7

C8

C9

C10

CMe

CMe

CMe

132.6 125.0 126.7 133.5 134.9 137.7 139.1 123.3

K3.8 K17.4 K15.1 K7.7 K5.5 K4.9 K3.3 3.8

107.6 107.3 109.7 118.3 120.7 120.5 123.1 106.9

128.6 127.2 129.2 130.4 132.4 130.6 132.7 128.5

144.9 141.9 144.9 142.9 145.7 143.0 145.9 141.9

127.2 125.7 126.7 126.6 127.5 127.1 127.9 123.9

126.8 126.1 126.9 126.7 127.4 127.4 128.1 122.6

127.5 125.2 125.9 126.4 127.0 127.3 127.8 124.5

130.4 130.4 130.8 133.1 133.1 134.0 134.2 130.7

35.0 30.1 31.7 29.8 31.3 27.7 29.3 37.9

29.7 29.2 29.5 29.1 29.3 29.6 29.8 31.9

29.7 26.1 26.3 26.7 26.8 26.9 27.1 29.8

29.7 26.1 26.3 26.7 26.8 26.9 27.1 29.8

7.2 6.4 5.2 5.7 6.4 7.5 5.9

2.2 1.4 2.0 1.9 2.1 2.2 1.9

The first bold number is the average error per carbon atom (ppm) for the cumulenic part, the second for the substituent R. a Ref. [7], solvent CDCl3.

which do not differ much within a given method, the exception being the alkyl iminopropadienone 1, where all theoretical shifts for the carbonyl carbon are found at a higher field. The central carbon atom C2 possesses relative shifts between C11 and K11 ppm, even K15 ppm in C3O2, and the CZN carbon atom signals vary by about 10 ppm. It is remarkable that none of the approaches tested in this study can describe the iminopropadienonic part for the range of molecules accurately. There are methods such as HF together with a small basis set for the NMR calculations based on a B3LYP or MP2 geometry which are good at reproducing the NMR spectra of 1–3, but have significant difficulties with the pentafluoroiminopropadienone 4. On the other hand, the well-evaluated approach B3LYP/6-31C G**//MP2/6-31G*, which has been employed successfully on several occasions [30,31], is by far the best method for

this compound. It very accurately describes 4, an electronically interesting molecule. However, this theoretical level systematically predicts the relative shift of the central carbon atom at lower field, while the imino and carbonyl atom signals appear at a higher field, with the exception of the C1 atom in 4. Similarly but even more pronounced, the B3LYP/6-31CG**//MP2/6-31G* calculation of the NMR properties for 5 give by far the most accurate results for the cumulenic part. It can hence be rated as the method of choice when dealing with iminopropadienones. Notably, the computationally most demanding MP2 GIAO calculation of 1 gives the worst average error for the cumulenic part of any system. The influence of the basis set can be seen from Table 1. At a given geometry, increasing the size of the basis set, e.g. going from 6-31G* to 6-31CG** leads to a small low field

Table 4 Experimental and calculated 13C NMR chemical shifts d (ppm) of 4, relative to TMS

a

Experiment HF/6-31G*//HF/6-31G* HF/6-31CG**//HF/6-31G* HF/6-31G*//B3LYP/6-31G* HF/6-31CG**//B3LYP/6-31G* HF/6-31G*//MP2/6-31G* HF/6-31CG**//MP2/6-31G* B3LYP/6-31CG**//MP2/6-31G*

C1

C2

C3

129.3 128.1 129.5 134.2 135.3 138.9 125.8 123.8

11.0 K8.1 K6.0 K2.9 K0.9 1.3 2.7 9.5

100.0 117.6 119.8 119.7 121.8 123.7 140.2 109.3

12.6 12.3 12.8 13.2 14.3 15.0 5.4

C4

C5

C6

C7

C8

C9

110.2 109.8 110.9 113.3 114.2 114.4 115.3 111.3

141.8 138.2 138.5 141.0 141.1 141.1 141.2 142.1

137.1 133.6 133.8 137.1 137.1 138.1 138.2 139.7

138.1 136.6 136.4 139.5 139.1 140.1 139.7 140.7

137.1 132.9 133.4 136.5 136.8 137.6 138.0 139.2

141.8 141.8 142.3 145.4 145.8 145.8 146.2 148.3

The first bold number is the average error per carbon atom (ppm) for the cumulenic part, the second for the substituent R. a Ref. [9], solvent CDCl3.

2.2 2.2 1.6 1.7 2.1 2.3 2.5

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R. Koch et al. / Journal of Molecular Structure (Theochem) 686 (2004) 31–36

Table 5 Experimental and calculated 13C NMR chemical shifts d (ppm) of 5, relative to TMS

a

Experiment HF/6-31G*//HF/6-31G* HF/6-31CG**//HF/6-31G* HF/6-31G*//B3LYP/6-31G* HF/6-31CG**//B3LYP/6-31G* HF/6-31G*//MP2/6-31G* HF/6-31CG**//MP2/6-31G* B3LYP/6-31CG**//MP2/6-31G*

a

Experiment HF/6-31G*//HF/6-31G* HF/6-31CG**//HF/6-31G* HF/6-31G*//B3LYP/6-31G* HF/6-31CG**//B3LYP/6-31G* HF/6-31G*//MP2/6-31G* HF/6-31CG**//MP2/6-31G* B3LYP/6-31CG**//MP2/6-31G*

C1

C2

C3

124.3 124.1 125.7 133.4 134.7 137.9 139.2 123.4

6.9 K20.1 K18.0 K7.8 K5.6 K4.5 K2.8 6.6

110.2 101.7 104.1 118.5 120.9 121.5 123.9 105.3

11.9 10.8 10.7 11.2 12.1 12.8 2.0

C4

C5

C6

C7

C8

C9

C10

C11

C12

C13

133.8 124.2 126.4 128.7 130.8 129.3 131.5 128.5

126.6 124.4 124.5 128.3 128.0 129.0 128.5 126.3

124.6 123.1 123.8 124.8 125.4 125.9 126.5 123.5

124.7 126.3 127.1 126.6 127.2 127.2 127.8 123.6

137.7 132.0 134.9 133.1 135.9 133.1 136.2 132.3

126.1 126.1 127.1 126.7 127.5 127.6 128.4 124.6

123.9 125.4 126.2 126.5 127.1 127.4 128.1 123.0

125.6 125.6 126.6 125.9 126.8 126.9 127.7 123.7

123.7 122.4 123.0 124.4 124.9 125.4 125.8 122.5

127.6 127.8 130.6 128.6 131.3 128.0 130.8 127.6

2.4 2.4 1.9 2.0 2.4 2.5 1.9

The first bold number is the average error per carbon atom (ppm) for the cumulenic part, the second for the substituent R. a Ref. [9], solvent CDCl3.

shift for the cumulenic signals (ca. 1–2 ppm) within the Hartree–Fock framework. The effect is more pronounced with B3LYP, where the shift is around 4 ppm. This is also apparent in Table 8. Also of interest is the role of the underlying molecular geometry. In Tables 1–5, there are entries for HF/6-31G* NMR calculations based on three different geometries. Again, the most pronounced effect is found for the NaCaCaCaO part of the molecule. One can easily see that the relative chemical shifts for C1, C2 and C3 are significantly affected by these geometries; all three signals appear at a lower field when going from HF to B3LYP to MP2 geometries. Table 6 lists the relevant structural data for 1. It seems that the dependency of C1 and C3 can, at least partially, be explained in terms of the changes in bond lengths of C1aO and C3aN. For these two bonds, the HF-calculated lengths are much shorter, and the corresponding chemical shifts differ significantly from those based on the other geometries. For instance, calculations of chemical shifts of the MP2/6-31G* structure with an artificially reduced CZO bond length by 2 pm to 117.2 pm leads to a change of the C1 shift from 130.1 to 126.1 ppm. The C2 signal is shifted also to higher field, but only to a lesser extent (from K8.9 to K11.1 ppm). However, differences in the geometries cannot sufficiently explain the calculated variations of the C2 signals. Changes of the NaCaC angle or the C2aC1 bond do not significantly affect the chemical shifts. Another attempt to rationalize the obtained results has been made employing the atoms-in-molecules (AIM) approach. Computed electron densities r for the three cumulenic carbon atoms for the iminopropadienones 1–5

are given in Table 7. The generally admitted interpretation of higher electron density at a specific atom resulting in chemical shifts at a higher field can be confirmed. For the carbonyl and the central carbon atoms C1 and C2, the calculated electron density correlates well with the theoretical chemical shift, i.e. a higher electron density leads to high-field shifts of d. The situation is more complicated for the imino carbon atom C3 due to the proximity and influence of the substituent R. For similar aromatic groups R (as in 2, 3 and 5), the correlation between r and d is valid, but fails for the aliphatic (in 1) and the electronically different substituent (in 4). These latter two chemical shifts also show the largest deviations from the experimental values. For instance, the large C3 electron density of 4 predicts a chemical shift to be more in the region of the experimental value of 100 ppm instead of the theoretical 109 ppm. The data of r and d for C3O2 give no correlation with the iminopropadienones. In general, AIM-calculated electron Table 6 Selected calculated bond lengths (pm) and bond angles (8) of 1, together with the calculated cumulenic 13C NMR chemical shifts d (at HF/6-31G*)

NaC3 C3aC2 C2aC1 C1aO NaC3aC2 C3aC2aC1 C2aC1aO dC1 dC2 dC3

HF/6-31G*

B3LYP/6-31G*

MP2/6-31G*

115.7 129.7 125.3 115.5 179.1 179.5 180.0 119.0 K24.0 98.9

120.1 129.1 127.2 117.9 175.0 179.7 179.9 126.1 K12.3 115.3

121.4 129.2 127.6 119.2 172.9 176.2 179.5 130.1 K8.9 118.2

R. Koch et al. / Journal of Molecular Structure (Theochem) 686 (2004) 31–36

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Table 7 AIM-derived electron densities r for the carbon atoms C1–C3 of 1–6 compared with calculated (B3LYP/6-31CG**//MP2/6-31G*) and experimental relative chemical shifts d (ppm) C2

C1

1 2 3 4 5 6

C3

r

d (theo)

d (exp)

r

d (theo)

d (exp)

r

d (theo)

d (exp)

5.1630 5.1333 5.1211 5.1208 5.1253 5.0989

115.4 121.8 123.3 123.8 123.4 113.2

130.0 132.5 132.6 129.3 124.3 129.7

5.6791 5.6758 5.6706 5.6191 5.6605 5.5755

K1.5 2.8 3.8 9.5 6.6 K8.9

K11.0 K6.8 K3.8 11.0 6.9 K14.6

5.2146 5.2213 5.2131 5.2518 5.2265

99.5 105.2 106.9 109.3 105.3

108.5 111.6 107.6 100.0 110.2

densities agree well with theoretical chemical shifts only for similar structures. Carbon suboxide 6 is a particularly challenging molecule (Table 8). There are three recent reports of calculated C3O2 13 C NMR data in the literature [32–34], which are also given in Table 8. As mentioned above, in the following we refer only to the first reported experimental data [10]. One of the most striking finding is the fact that all DFT methods, even those with very large basis sets, have more or less severe problems getting the chemical shifts correct. It appears that these calculations are systematically erroneous; most of the C1 shifts are too small. The C2 signals vary by 5–13 ppm around the experimental value. Extended basis sets and/or other functionals seem to improve this behavior but still give too low values for C1. The best results were obtained with HF NMR calculations with small or medium basis sets using B3LYP and MP2 geometries. This is not surprising, as Table 8 Experimental and calculated 13C NMR chemical shifts d (ppm) of C3O2 6, relative to TMS

Experiment (1975) HF/6-31G*//HF/6-31G* HF/6-31CG**//HF/6-31G* HF/6-311CG(3df,2p)//HF/6-31G* HF/6-31G*//B3LYP/6-31G* HF/6-31CG**//B3LYP/6-31G* B3LYP/6-31G*//B3LYP/6-31G* B3YLP/6-31CG**//B3LYP/6-31G* B3LYP/6-311CG(3df,2p)//B3LYP/6-31G* HF/6-31G*//MP2/6-31G* HF/6-31CG**//MP2/6-31G* HF/6-311CG(3df,2p)//MP2/6-31G* B3LYP/6-31G*//MP2/6-31G* B3LYP/6-31CG**//MP2/6-31G* VWN/“TZ2P”//BP86/TZPa BP86/“TZ2P”//BP86/TZPa VWN/VII(ADF)//BP86/TZPb BP86/VII(ADF)//BP86/TZPb SAOP/“TZ2P”//BP86/TZPc KT2/“TZ2P”//KT2/TZPc

C1

C2

129.7 122.6 124.4 136.3 125.8 126.8 108.8 112.8 125.2 131.2 132.7 144.7 112.7 113.2 123.5 122.8 123.3 122.5 121.1 122.2

K14.6 K24.8 K24.6 K27.8 K18.4 K16.7 K18.1 K15.6 K18.4 K15.6 K15.6 K17.9 K11.0 K8.9 K11.7 K10.1 K16.4 K15.6 K13.4 K12.6

no current functionals include a magnetic field dependence, and so the DFT methods do not provide systematically better NMR results than Hartree–Fock approaches.

4. Conclusion To summarize, it is still a challenge to precisely calculate C NMR chemical shifts for multiply cumulated double bond systems. The best approach is to use the B3LYP/631CG**//MP2/6-31G* level of theory. It reproduces iminopropadienone substituents extremely well, and the underestimation of the cumulenic carbon chemical shifts is systematic by usually 5–10 ppm. The HF/6-31C G**//B3LYP/6-31G* level of theory is computationally more efficient because it uses only a DFT geometry, which is the time limiting step. This level appears in several cases to give a superior description of the cumulated double bond system but has to be evaluated on a case-by-case basis. 13

Acknowledgements 8.1 6.9 8.8 3.9 2.6 15.1 11.6 4.3 1.3 2.3 11.1 12.5 12.9 5.1 6.1 4.9 5.1 6.1 5.7

The bold number is the average error (with respect to the 1975 experiment [10]) per carbon atom (ppm). a Ref. [32]. b Ref. [33]. c Ref. [34].

This work was supported by generous allocation of computer time at the Norddeutscher Verbund fu¨r Hoch- und Ho¨chstleistungsrechnen (HLRN), Hannover, Berlin and at the Hochschulrechenzentrum, Universita¨t Oldenburg. Financial support by the Australian Research Council and the Deutsche Forschungsgemeinschaft is gratefully acknowledged.

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