2.5 instrumental analysis Flashcards
what is the empirical formula?
the simplest whole number ratio of the elements found in a compound
what are the steps to working out the empirical formula?
1) mass of each element
2) divide each by its GFM
3) divide each by the smallest number
what does mass spectrometry tell us about?
the structures within a molecule
how are fragments formed in mass spectrometry?
- sample bombarded with high-energy electrons causing electrons to be knocked off
- forms positive ions which break down into smaller fragments
how are the fragments recorded in mass spectrometry?
deflected onto a detector that records the mass/charge ratio of the fragment.
in mass spec, what s the parent ion?
the peak with the highest m/z
what does the parent ion tell us about?
the gfm of the compound
what is infrared spec used to identify?
certain functional groups in a compound
IR radiation is passed though a compound in an IR spectrometer. Depending on the … , …
-type of bonds present, different wavelengths of radiation are absorbed
what does an IR detector observe?
the intensity of radiation absorbed by a sample at different wavelengths
what is the absorbance of IR measured in?
wavenumbers (cm^-1)
IR radiation can cause parts of a molecule to …
vibrate
name the types of vibration that occur as a result of IR radiation
stretching, bending, scissoring
different vibrations …
absorb different wavelengths of IR
the wavelength of IR absorbed depends on:
the strength of the bond
the type of atoms attached to the bond
from which region of IR spec should we try to identify peaks from?
above 1500cm^-1
what information does proton NMR give us?
info about different chemical environments of hydrogen atoms and how many hydrogen atoms are in each of these environments
when exposed to a powerful magnet, hydrogen atoms either:
align themselves with the field (lower energy),
or align themselves against the field (higher energy).
describe the process of producing an NMR spectrum
Absorption of radiation in the radio frequency causes the H nuclei to ‘flip’ from
the lower to the higher energy alignment.
As they fall back from the higher to the lower
the emitted radiation is detected and plotted on a spectrum.
the amount of energy released by hydrogens falling to low energy state is affected by…
the environment of the H atoms
what is the standard reference substance used for proton NMR?
tetramethylsilane (TMS)
how many proton environments does TMS have?
1
what is the chemical shift value of TMS?
0
which page of the data book gives chemical shift values for different proton environments?
page 17
what does low-resolution proton NMR show?
the environments of the peaks and the number of hydrogens at each environment
what does high-resolution proton NMR use?
higher radio frequencies than low resolution and produces more detailed spectra
what does high-resolution NMR show us?/
how many H neighbours each environment has
list everything that low res proton NMR tells us
- the number of different environments that H is found in (no. of peaks - ref)
- the chemical environment of the hydrogens (see chemical shift values on pg 17)
- the number of H atoms in each environment (relative heights)
what does a doublet mean?
there is 1 H atom on the adjacent C
what does a triplet mean?
there are 2 H atoms of the neighbouring c
