|Chapter 13: Spectroscopy|
The number of protons in an atom. Electron configuration The arrangements of electrons above the last (closed shell) noble gas. Br–Br 192.8 Br 2: C–Br 285 general Br–H 365.7. (1 u is equal to 1/12 the mass of one atom of carbon-12) Molar mass (molar weight) is the mass of one mole of a substance and is expressed in g/mol. Weights of atoms and isotopes are from NIST article. Give us feedback about your experience with Molecular Weight Calculator. Related: Molecular weights of amino acids: molecular weights calculated. According to quantum-chemical reaction path calculations, the bond breakage proceeds after the intrusion of a bromine atom into the B–H σ-bond. The 9-Br and 9-OH substituents in carborane 1 strongly retard the bromination of the corresponding derivatives.
Isotope patterns for -Cl and -Br Installing pirated software on mac.
- Mass spectrometers are capable of separating and detecting individual ions even those that differ only by a single atomic mass unit (note in rality they are far more sensitive than that!)
- As a result, molecules containing different isotopes can be distinguished.
- This is most apparent (at this level) when atoms such as bromine or chlorine are present in a molecule because those elements naturally exist with a significant % of the heavier isotope.
- For example, while C has 2 common isotopes, 12C and 13C, 13C represents only about 1% of natural carbon. In contrast, Cl has 2 common isotopes, 35Cl and 37Cl, with about 25% being 37Cl.
- Typically, one looks at the molecular ion peak, 'M' (since this is being identified and used to determine the MW).
- When working with MW from the molecular ion in MS, the best approach is to always use the lighter ion (i.e. M) and the mass of the lighter isotope (i.e. for Cl use 35 not 35.5, or, for Br use 79 and not 80)
Br Atom Mass
- 35Cl : 37Cl exists naturally in an almost 3:1 ratio, so we observe peaks at 'M' (molecules with an atom of 35Cl) and 'M+2' (molecules an atom of 37Cl) are obtained with relative intensity 3:1
- 79Br : 81Br exists naturally in an almost 1:1 ratio, so we observe peaks at 'M' (molecules with an atom of 79Br) and 'M+2' (molecules an atom of 81Br) are obtained with relative intensity 1:1
Br Atom Number
- Note that since the relative natural abundances of the isotopes are different, you can tell the difference between the presence of Cl and Br. The patterns are different, they look different.
- 'M+1' peaks are usually seen due to the presence of 13C in the sample but because 13C is only about 1% of natural carbon, the peaks tend to be small (unless there is a large number of C atoms present). Note that you can see the small peaks due to the presence of 13C in the figures shown for Cl and Br, they look like little shadows on the right of the other peaks.
The following two examples of mono-haloalkanes mass spectra show the characteristic isotope patterns of monohalogenated molecules. The patterns are highlighted in the green boxes:
Example 1 :
This MS is of 2-chloropropane, C3H7Cl.
Note the characteristic isotope pattern at 78 (M) and 80 (M+2) in a 3:1 ratio.
Loss of 35Cl from 78 or37Cl from 80 gives the base peak a m/z = 43 (M - 35 = M+2 - 37 = 43) corresponding to the secondary propyl cation.
Note that the peaks at m/z = 63 and 65 represent fragment ions that still contain Cl and therefore also show the 3:1 isotope pattern.
The very small peak at 79 represents M+1, the small number of molecules that contain 35Cl and an atom of 13C rather than 12C.
The even smaller peak at 81 presents M+2+1 = M+3, a very small number of molecules that contain 37Cl and an atom of 13C rather than 12C.
Br Atomic Size
Example 2 :
This MS is of 1-bromopropane, C3H7Br.
Note the isotope pattern at 122 and 124 represents the M and M+2 in a 1:1 ratio.
Loss of 79Br from 122 or 81Br from 124 gives the base peak a m/z = 43, corresponding to the propyl cation.
Note that other peaks, such as those at m/z = 107 and 109 (yes, they are small) still contain Br and therefore still show the 1:1 isotope pattern.
Note: the isotope patterns for polyhalogenated molecules (such as having both -Cl and -Br or with multiple -Cl or -Br) give different (but still characteristic isotope patterns).
Br Atom Radius
|© Dr. Ian Hunt, Department of Chemistry|