Double-bond equivalency

From Cg/ZgNO (problem 4), for example, the empirical formula C Hjo is derived and compared with the alkane formula Cc>H2o, a hydrogen deficit of ten and thus of five double-bond equivalents is deduced. If the NMR spectra have too few signals in the shift range appropriate for multiple bonds, then the double-bond equivalents indicate rings (see, for example, a-pinene. Fig. 2.4). 

The empirical formula contains five double-bond equivalents. In the H NMR spectrum a doublet signal at Sh = 9.55 stands out. This chemical shift value would fit an aldehyde flinction. Since the only oxygen atom in the empirical formula is thus assigned a place, the methyl signal at Sh = 3.80 does not belong to a methoxy group, but rather to an /f-methyl group. 

Apart from the A-methyl group, three double-bond equivalents and three multiplets remain in the chemical shift range appropriate for electron rich heteroaromatics, Sh = 6.2 to 6.9. A-Methyl-pyrrole is such a compound. Since in the multiplets at Sh = 6.25 and 6.80 the Jhh coupling of 4.0 Hz is appropriate for pyrrole protons in the 3- and 4-positions, the pyrrole ring is deduced to be substituted in the 2-position. 

The //NMR spectrum contains five signals with integral levels in the ratios 1 1 1 1 3 four lie in the shift range appropriate for aromaties or heteroaromaties and the fifth is evidently a methyl group. The large shift values (up to Sh = 9.18, aromaties) and typical coupling constants (8 and 5 Hz) indicate a pyridine ring, which accounts for four out of the total five double-bond equivalents. 

The substruetures A - D aeeount for eleven of the total of twelve double bond equivalents so that the eompound eontains one additional ring as present in anthraeene or phenanthrene skeletons. 

Assembly of fragments A-C, taking into aecount the ninth double-bond equivalent, leads to the 3,5,7,3, 4 -pentahydroxyflavane skeleton D and to the following assignment of H chemieal shifts ... 

The long-range coupling of 2.2 Hz which appears in A and B, two quaternary C atoms in the NMR spectrum with appropriate shifts (5c = 76.6 and 83.0) and the two double-bond equivalents (molecular formula C TZ/oO) suggest that a CC triple bond links the two structural fragments. Hence the compound is identified as hex-3-yn-l-ol (C) in accordance with the coupling patterns. 

Five double-bond equivalents can be recognised from the shift values (four for the benzene ring and one for the carbonyl group). The sixth double-bond equivalent implied by the molecular formula belongs to another ring, so that the following pieces can be drawn for the molecular jigsaw puzzle ... 

The molecular formula Ci8/f220n contains eight double-bond equivalents, i.e. four more than those in the framework 1 known to be present. The C NMR spectrum shows two carboxy-CO... 

Internal E- or Z-double bonds or double bond equivalents can be disconnected. 

In this book, in order that you can concentrate your attention on the NMR spectra, we shall provide you with the molecular formula in all cases. This in turn provides you with information which can be extremely useful during the process of solving the structure if the molecule only contains C, H, N and O then you can use the molecular formula to obtain the number of so-called double bond equivalents, i.e. information on the degree of unsaturation. Though there are various formulas which can be devised to do this, we recommend the calculation using the following formula for a molecule CaHbOcNd, the number of double bond equivalents DBE is calculated as follows... 

We need to define what is meant by a double bond equivalent any element-element double bond (C = C, C = 0, C=N) count as 1, while triple bonds count as 2. A saturated ring counts as 1, and any double bond present in the ring also counts as 1 thus a benzene ring corresponds to 4 double bond equivalents. 

First use the molecular formula and the equation given above to calculate the number of double bond equivalents. In this case (remembering to treat bromine as equivalent to hydrogen) the value is 1. The infrared spectrum shows a band at 1641 cm 1, which probably represents the C=C bond stretch, but in this case there can only be a C=C bond present ... 

Index of Hydrogen Deficiency (degree of unsaturation, the number of double-bond equivalence) ... 

FIGURE 7.4 The PCA score plot of mass spectral data from ketones shows clustering by the number of double bond equivalents (indicated by the numbers 1-5). Variables used are 14 autoscaled modulo-14 features. 

Pseudopterosin X (1) was isolated as a yellow colored gum. The UV spectrum of 1 showed maximum absorption at 280 nm due to the presence of a highly substituted benzene chromophore [10], Its IR spectmm displayed intense absorption bands at 3,470 (OH), 2,904 (CH), 1,705 (C = O), 1,595 (C = C) and 1,100 (C-0) cm . The high-resolution electron-impact mass spectmm (HREIMS) of 1 showed M+ at m/z 474.2622, and this mass provided molecular formula indicating the presence of nine double bond equivalents in 1. The C-NMR chemical shift assignments of 1 are shown around stracture 1. On the basis of the detailed NMR studies and comparison with the reported pseudopterosins in the literature and L-xylose [3-5], stmcture 1 was proposed for this new natural product. 

Micaceol (12) was isolated as a light yellow amorphous solid. Its UV spectram showed maximum absorption at 246 mu indicahng the presence of a conjugated n system. The IR spectmm exhibited intense absorption bands at 3,425 (OH) and 1,601 (C = C) cm". The Chemical Ionization Mass Spectrum (CI-MS) of 12 showed molecular ion peak at m/z 399 [M-H]+. The HREIMS of this compound showed molecular ion peak at m/z 398.2392, which provided the molecular formula (calcd = 398.2360) and indicated the presence of six degrees of unsatura-hon in 12. These six double bond equivalents were accounted for by the steroidal skeletons with two double bonds incorporated in its structure. 

Build search criteria that include information such as possible elements and the numbers of each element maximum and minimum number of double bond equivalency (DBE) maximum tolerance of experimental error in the determined exact mass. 

Accurate mass measurement and the associated empirical formulae allow routine calculation of the double bond equivalent (DBF) by the spectrometer s computer system. This is a measure of the number of double bonds and/or the number of rings in a molecule, and is derived from a consideration of the valences of the various elements in a given composition. This gives information about the aromaticity or conjugation of the unknown. The values given by the MS data system are based on a simple calculation ... 

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