ChemDraw User's Manual - CambridgeSoft

Appendix D:WorksUltraHow ChemNMROverviewChemNMR estimates chemical shifts for allhydrogen or carbon atoms for which additivity rulesare available. Following a hierarchical list, it firstidentifies key substructures of a molecule. Asubstructure provides the base value for theestimated shift. For example, benzene would beidentified as the key substructure of trinitrotoluene.When a substructure happens to be a ring systemnot available in the data, ChemNMR approximatesits base shift using embedded rings and, ifnecessary, it will even disassemble the ring intoacyclic substructures.ChemNMR views remaining parts of the moleculeas substituents of a substructure. Each substituentadds to or subtracts from the base shift of thesubstructure to which it is attached. Additivity rulesdetermine the increment of each contribution. If anincrement for a substituent cannot be determined,ChemNMR uses embedded substituents—smallerstructural units with the same neighboring atoms.Or, it will use increments of identical or embeddedsubstituents of a corresponding substructure byassuming that the effects of the substituents are ofthe same magnitude.ChemNMR provides a detailed protocol of theestimation process applied. It gives substructures asnames, compound classes in most cases,substituents in form of a linear code, respectively.The data set for the 1 H NMR Shift tool currentlycontains 700 base values and about 2000increments. The 13 C NMR Shift tool is based on4000 parameters. It also implements models forethylenes (cis/trans) and cyclohexanes(equatorial/axial).ChemNMRLimitationsThe program handles the following elements:H, D, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si,P, S, Cl, Ar, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu,Zn, Ga, Ge, As, Se, Br, Kr, Rb, Sr, Y, Zr, Nb, Mo,Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, I, Xe, Cs, Ba,La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, Yb, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl,Pb, Bi, Po, At, Rn, Fr, Ra, Ac, Th, Pa, U, Nep, Pu,Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr. Functionalgroups are expanded automatically.In case of 1 H NMR, it estimates shifts of about 90%of all CH x -groups with a standard deviation of0.2. 0.3 ppm. The use of polar solvents maystrongly increase these deviations. It does notestimate shifts of hydrogen atoms bonded toheteroatoms because they are significantly affectedby solvents, concentration, impurities, and stericeffects.In case of 13 C NMR, it estimates over 95% of theshifts with a mean deviation of -0.29 ppm andstandard deviation of 2.8 ppm.For more information see:Fürst, A.; Pretsch, E. Anal. Chim. Acta 1990, 229, 17.Pretsch, E.; Fürst, A.; Badertscher M.; Bürgin, R.;Munk, M. E. J. Chem. Inf. Comp. Sci. 1992, 32, 291-295.ChemDraw 9 How ChemNMR Works • 243ChemNMR Limitations