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ACD/Labs Blog

Arvin Moser
MS and NMR are complementary tools for structure elucidation. Knowing when to apply which tool can assist an elucidator in solving for an unknown structure quickly and with less frustration. Although the sample data below is for a simple organic structure, it is working through many simple examples that one refines the skills needed for...

Arvin Moser
Although NMR offers some strategies for determining the presence of a fluorine atom(s), fragment loss of a CF3 group can be determined from a mass spectrum. The EI MS for 2,2,2-trifluoro-N,N-dimethylacetamide (Mmi = 141 Da) is shown below. The ion peak at m/z 72 indicates the loss of a CF3 group. Although the ion peak...

Arvin Moser
Without correctly identifying the molecular ion on a mass spectrum, the nitrogen rule generally cannot be applied to any arbitrary ion peak. The chemical structure for N-[(1E)-1-phenylethylidene]methanamine and its EI mass spectrum are shown below. As expected, the odd value for the molecular ion at m/z 133 dictates an odd number of nitrogen atoms. However,...

Arvin Moser
For MS data, the ionization method will dictate how the nitrogen rule is applied. For example, if the ionizer is Electron Impact (EI) ionization, then the nitrogen rule is to be applied to the molecular ion [M]+* as follows: -an odd nominal mass indicates an odd number of nitrogen atoms, e.g. 1,3,5 -an even nominal...

Arvin Moser
The purpose of the nitrogen rule is to assist with deciphering how many nitrogen atoms are present without any prior information on the molecular formula. Depending on the ionization mode, an odd nominal mass indicates an odd number of nitrogen atoms, e.g. 1,3,5, whereas an even nominal mass indicates an even number of nitrogen atoms,...

Arvin Moser
MS and NMR are complementary elucidation tools. Knowing when to apply the correct tool can facilitate the elucidation process. Compounds with bromine atoms exhibit a distinct ion pattern on a mass spectrum. The A+2 peak for a monobrominated compound appears at almost identical intensity to the 79Br peak due to the presence of 81Br (~49.3%...

Arvin Moser
A calculated neutral loss spectrum is obtained from a mass spectrum by determining the mass differences between the precursor ion m/z and each of the other peaks in the spectrum and plotting the original intensity versus neutral mass. Neutral losses with small masses have limited possibilities for their composition and thus can facilitate the identification...

Arvin Moser
Without any structural information, we can estimate an upper limit on the number of carbons using the m/z for the molecular ion. For the molecular ion at m/z 386.4 Da (shown in the mass spectrum below), the upper limit on the number of carbons is calculated at 386.4 / 12 = 32.2. Rounding down, we...