ch 11 - Spectroscopy Flashcards
spectroscopy
measures energy differences between possible states of a molecular system by determining the frequencies of electromagnetic radiation absorbed by the molecules
Infrared (IR) spectroscopy
measures molecular vibrations, which can be seen as bond stretching, bending, or combinations of different vibrational modes; infrared light is passed through a sample, and the absorbance is measured; infrared light range runs from wavelength=700 nm to 1mm (useful absorptions occur at 2500 to 25,000 nm
wavenumber
1/wavelength; an analog of frequency used on IR spectrum; standard range corresponding to 2500 to 25,000 nm is 4000 to 400 cm^-1 (note: frequency=speed of light/wavelength)
fingerprint region
range of 1500 to 400 cm^1 wavenumber in IR spectrum where we can see more complex vibration patterns caused by motion of the molecule as a whole; called this because the specific absorbance pattern is characteristic of each ind molecule; this range is outside scope of mcat
to read vibration for IR spectroscopy
there must be a change in bond dipole moment (molecules that are symmetrical or are composed of atoms with same electronegativity do not exhibit absorption and are silent)
IR absorption for hydroxyl group
O-H absorbs with a broad (wide) peak at around 3300 cm^-1 for alcohols and 3000 cm^-1 for carboxylic acids
IR absorption for carbonyl group
C-O absorbs around 1700 cm^-1 with a sharp (deep) peak
IR absorption for amine group
N-H absorbs around 3100-3500 cm^-1 and has sharp peak
transmittance
amount in percent of light that passes through the sample and reaches the detector vs. wavenumber in IR spectroscopy; max absorptions appear as the bottom of valleys on spectrum
UV spectroscopy
need to know: obtained by passing UV light through a sample usually dissolved in inert, nonabsorbing solvent, and recording absorbance which is then plotted against wavelength; absorbance caused by electronic transitions between orbitals; gather info on wavelength of max absorbance, which tells extent of conjugation - more conjugated the lower energy of transition and greater wavelength of max absorbance
UV spectroscopy use
most useful for studying compounds with double bonds and/or heteroatoms with lone pairs that create conjugated systems
easily excited molecules in UV spectra
have lower energy gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO); absorb longer wavelengths (lower frequencies) with lower energy
Nuclear Magnetic Resonance Spectroscopy (NMR)
most important spectroscopic technique for test: based on the fact that certain atomic nuclei have magnetic moments that are oriented at random; when placed in magnetic field moments tend to align with or against direction of field
alpha-state nuclei
nuclei with magnetic moments that are aligned with the magnetic field they’ve been placed in; they are lower energy in NMR
beta-state nuclei
nuclei that have been irradiated with radiofrequency pulses that match the energy gap between the two states which will excite some lower-energy (alpha-)nuclei into this higher energy state
How NMR spectrum works
frequency vs. absorption of energy; uses arbitrary variable called chemical shift (lower case delta), with units of parts per million (ppm) of spectrometer frequency; chem shift plotted on x-axis and increases toward left (called downfield); tetramethysilane (TMS) is used as a calibration standard to mark 0 ppm to show just how far downfield compounds are
^1H-NMR
NMR studies absorption of protons and only ^1H are studied on the mcat (hydrogen)
chemically equivalent
in H-NMR means that multiple protons have the same magnetic environment and will lead to the same peak; Hs attached to different groups on a molecule have different peaks from other groups’ Hs but the same peak as each other - specifically Hs as protons
integration
area under the peak of an H-NMR spectra; size of peak is proportional to number of protons it reflects
deshielding
act of atoms pulling electron density away from H atom in H-NMR spectroscopy, deshielding it from the magnetic field (causes it to be further away from the TMS peak); the more density is pulled away, the less it can shield itself from the applied magnetic field, resulting in a reading further downfield
spin-spin coupling (splitting)
occurs when two or more hydrogens are three bonds away from each other like H-C-C-H
doublet
number of peaks that occur in H-NMR when two hydrogens are in close proximity (not attached to O or N)
n + 1 rule
how to determine how many peaks to have in one spot on H-NMR spectroscopy; if a proton has ‘n’ number of protons that are three bonds away, it will be split into n + 1 peaks
coupling constant, J
magnitude of splitting in H-NMR
main shifts to know on H-NMR
alkyl groups: 0-3 ppm; alkynes: 2-3 ppm; alkenes: 2-3 ppm; aromatics: 6-8.5 ppm; aldehydes: 9-10 ppm; carboxylic acids: 10.5-12 ppm
