presence of aliphatic groups, or unsaturated or aromatic structures may be
inferred from the position of the −C-H stretching bands around 3000 cm−^1 ,
and confirmed by the presence of other bands. A useful application of Raman
spectrometry (Topic E10) is the detection of groups that have very weak
absorbances in the infrared region, such as substituted alkynes.(iv) The UV spectrum does give some structural information, even when there
is little or no absorbance, which would suggest the absence of any aromatic,
conjugated or ketonic structures. If there are double bonds or unsaturated
rings present, the UV spectrum should provide further information.(v) Much useful information may be derived from the mass spectrum as dis-
cussed fully in Topic E14. A brief summary of what to look for should include:● the m/zof the molecular ion. This corresponds to the molecular
formula, which may be a multiple of the empirical formula derived in
(i). An odd value for the m/zof the molecular ion requires that an odd
number of nitrogen atoms are present, as in the example in (i) above.
Prominent isotope peaks indicate the presence of Cl, Br or S.
● the exact value of m/z for the molecular ion. For example, the nominal
RMM of the example in (i) is 175, and the formula might have been
deduced if the exact mass was determined as 175.0998, since, excluding
some impossible formulae, some others are:C 8 H 5 N 3 O 2 175.0382
C 7 H 13 NO 4 175.0845
C 11 H 13 NO 175.0998
C 10 H 13 N 3 175.1111● the fragments present and the fragments lost.(vi) Both the 1-H NMR and the13-C NMR give essential information about the
types of protons and carbons present, their environment and their connec-
tions to neighboring atoms. This is discussed in detail in Topics E12 and
E13.(vii) Before the final report is given, it is always a good idea to retrace the steps
above to check whether the data is self-consistent. For example, if there is
no evidence for aromatic structures in the IR spectrum, is this consistent
with the NMR spectrum? If an isomer must be identified, do the positions
of the peaks in the IR and NMR spectra correspond, and does the fragmen-
tation in the mass spectrum provide confirmation?F2 – Sample identification using multiple spectrometric techniques 287