mlz and intensitv for the six most intense peaks in the mass spectrum. M1 is the base (most intense) ~ e s and k is assumed t o have an intensitv of 100. The maximum MW and/or M is 255. RAM locations 38398 and 38399 contain the number of mass spectra in the lihrary. BIGSIX CATALOG contains the compound names for each mass spectrum and is accessed by BIGSIX a t the completion of a da.ta search. It is n random access data file with a maximum of :32 characters available fir each rompound name. This type of file, although more costly in terms of storage space, is considerably faster to search than a corresponding sequential data file. ~t present there is no encoding of data to increase the stored name length, apart from the internal use of "%" for "." which is rewired to overcome a data readine ~ r o h l e mwith library Applesoft BAQIC.The maximum size of the BIGSIX that can he accommodated on a single disk is 2000. BIGSIX will list all the relevant operating instructions. I t is not necessary to normalize the unknown mass spectrum, hut the base peak must have an intensity less than 256. There are a number of checks made for this and other input errors, with appropriate messages displayed if invalid data is entered. T o minimize data entrv ~rohlems.onlv valid characters are entered and displayed on the terminal screen. Following the data search, the computer will list the names of the three most probable compounds from the library, together with their molecular weight and percent similarity to the unknown mass spectrum. Figure 8 shows a typical run; all data entered from the terminal is shown underlined. In addition to the BIGSIX search program there is a utility EDIT program (approximately 520 statements) that enables the creation and updating of the BIGSIX lihrary. I t is also
possible to obtain a complete or partial listing of the library on either the video terminal or a hard copy printer. Users with only a single disk drive can use EDIT to back up the BIGSIX lihrarv on another disk. The followine is a list of the five single-key commands available: A-append spectra to the library B-backuo the librarv on another disk I.-list part or all o t t h e l i h r ~ r y M-mcrdtfg the ex~rtinglibrary h x i t to Applesoft A copy of the APPLESEARCH programs, together with a sample library of 1800 mass spectra, can be supplied on a 5-in. diskette for $30 (Australian). Please address all correspondence to the author a t the above address.
Mass Spectra of Organic Compounds Containing Bromine and Chlorine David K. Holdsworth University of Papua New Guinea Papua New Guinea A student ran use a pocket programmnhle calculator to analvze the data available from the isotooic oeaks of the low resoiution mass spectrum of a halogeno-organic compound. The number of atoms of bromine and chlorine can be determined together with the composition of the rest of the molecule. Mass spectroscopy enables the molecular formula of an o ~ a n i com~ound c containing carbon, hydrogen, nitrogen, and oxygen to he found by examination of either the high resolution mass of the molecular ion M or the relative ahundances of isotopic peaks M + 1and M + 2. A programmable calculator can he used to display, for a given integer mass, all possible formulas together with their high resolution masses and approximate percentage isotopic peaks (21). When a n oreanic molecule contains hromine andlor chlo" rine atoms, the relative ahundances of the isotopic peaks are drastically changed. Chlorine has two stable isotopes W l and W 1 in the ratio of about 31and bromine has isotopes 79Brand 81Br in approximately equal ratios. The mass spectrum of a compound containing one chlorine atom will have two molecular peaks with a n abundance ratio approximately 31,two mass units apart, corresponding to (R 3 W ) + and (R 37Cl)+. Similarly, the mass spectrum of a monohromo compound will show two molecular peaks of approximately equal height, two mass units aoart. . . corresoondine" to (R . 79Br)+and (R alBr)+. These patterns can he readily recognized in a massspectr&u and used to indicate a monochloro or monohromo com~ound. The intensity ratio of molecular-ion clusters of halogen isotopes may be calculated by means of the hinomial expansion (a b)", where a is the abundance of the lighter isotope, b the ahundance of the heavier isotope and n the number of atoms of the halogen. When a combination of hromine and chlorine atoms is present in a molecule, the intensitv ratio of the molecular-ion clusters are the products of the binomial expansions for bromine and chlorine. The general equation is
+
(a
+ b)" (C + d f m
where a = 0.5057, b = 0.4943, c = 0.754 and d = 0.246. In a molecule containing one chlorine atom (m = 1) and one hromine arum ( n = 1 ) the relative probability of occurrence of isotooic peaks at M, A1 + 2, M + 4 ran be ralrulnted. In this case thkre-are three isotopic peaks (m + n + 1).and their relative heights when plotted show the distinctive pattern noted in the mass spectrum of an organic compound containing one atom of bromine and chlorine. For example, (a
780
Journal of Chemical Education
+ b)' + d)' (C
= oe +ad
+ be + bd
isotopic isotope combinations peaks represented calculation M
35C170Br
(0.754X 0.5057)
M+2
35C18'8r
(0.754 X 0.4943)
% of M
0.381
100
0.497
130
0.122
32
+(0.246 X 0.5057) (0.246 X
+3'Cl'9Br s'CI"Br
M+ 4
relative probability of occurrence of peaks
n asam
-
Calculations involvine more haloeen atoms become lenethv - . and very tedious. A pocket programmable calculator can be used to display the relative heights of the M, M 2, M 4, etc., isotopic peaks (Figure 9). The probability of obtaining a combination of mass (M 22) where M is the combination containing only 35Cl and 79Br isotopes is given (22) by
+
+
+
C,-,"
PM+zz= i
X
C L mX on-"+' X bx-I
X em-' X d'
=o
A calculator with a peripheral printer with an automatic plotting capability, such as the Hewlett-Packard HP-41C, enables the relative heights of the isotopic peaks of a molecule containing any combination of bromine and chlorine atoms to be printed directly (Figure 9). Thus the student can immediately identify the number of bromine and chlorine atoms in a molecule by inspection of its mass spectrum. Despite the predominance of the M , M + 2, M 4, etc., isotopic peaks in the spectra of bromo- and chloro-organic compounds the (M 1)lM ratio is not affected by the presence of either chlorine or bromine and this ratio may still be used to indicate the C, H, N and 0 composition of the remainder of the molecule. The M 1oeak is related to the number of '3C, 2H, i 6 and ~ ' 7 0 isotopes in the molecule, esoeciallv those of 13C.A Docket ~roerammablecalcUht0r has been useb to display the approxhatk percentage (M + l)/M,
+
Table 1.
+
data from mass spectrum isotopic peak abundance M (235) M+ 1 M+2
abundance percentage of M
calculator display 100%
M+3
31.5
M+4 M+ 5
110.0
100.0 7.0 130.0 9.2 32.1
7.8
2.3
345.0 24.0
450.0
lor BrCl
130% 32%
cab~lalorentry: CHNO fragment mass: 121 (M- 114) NN-: 3 Nhalwenr: 2
.r C1,
+
NH < 2 NC + NN+ 2 - NmWena N~
