Alkanes do not ordinarily exhibit a molecular ion M+.M +. in the electron ionization mass spectrum because

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Alkanes do not ordinarily exhibit a molecular ion M+.M+. in the electron ionization mass spectrum because M+.M+. breaks into fragments. If dilute alkane vapor in HeHe is allowed to expand through a narrow orifice into a vacuum, expansion of the gas cools the molecules and reduces their vibrational energy. The molecular ion of the alkane C72H146 С72Н146 is the base peak when the molecule is sufficiently cold.

a. Find the nominal mass of the molecular ion C72H146+. С72Н146+.

b. Find the monoisotopic mass of C72H146+. С72Н146+.

c. 13C13Chas a natural abundance of 1.07%,1.07%, whereas 2H2H has a natural abundance of 0.012%.0.012%.

Therefore, the main isotopologues of the molecular ion are 12C72H1461 12C72H146 and 12C7113C1H146. 12C7113C1H146. As in the previous problem, we can find the relative abundance of 12C7212C72 and 12C7113C112C7113C1 from the first two terms of the binomial expansion (a+b)72,(a+b)72, where a=0.989 3 a=0.989 3 for 12C 12C and b=0.010 7b=0.010 7 for 13C13C. Predict the relative abundance of MM and M+1M+1 in the mass spectrum of C72H146.C72H146.

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Quantitative Chemical Analysis

ISBN: 9781319164300

10th Edition

Authors: Daniel C. Harris, Charles A. Lucy

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