ch 11 - Spectroscopy

Question 1

spectroscopy

Answer 1

measures energy differences between possible states of a molecular system by determining the frequencies of electromagnetic radiation absorbed by the molecules

Question 2

Infrared (IR) spectroscopy

Answer 2

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

Question 3

wavenumber

Answer 3

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)

Question 4

fingerprint region

Answer 4

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

Question 5

to read vibration for IR spectroscopy

Answer 5

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)

Question 6

IR absorption for hydroxyl group

Answer 6

O-H absorbs with a broad (wide) peak at around 3300 cm^-1 for alcohols and 3000 cm^-1 for carboxylic acids

Question 7

IR absorption for carbonyl group

Answer 7

C-O absorbs around 1700 cm^-1 with a sharp (deep) peak

Question 8

IR absorption for amine group

Answer 8

N-H absorbs around 3100-3500 cm^-1 and has sharp peak

Question 9

transmittance

Answer 9

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

Question 10

UV spectroscopy

Answer 10

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

Question 11

UV spectroscopy use

Answer 11

most useful for studying compounds with double bonds and/or heteroatoms with lone pairs that create conjugated systems

Question 12

easily excited molecules in UV spectra

Answer 12

have lower energy gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO); absorb longer wavelengths (lower frequencies) with lower energy

Question 13

Nuclear Magnetic Resonance Spectroscopy (NMR)

Answer 13

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

Question 14

alpha-state nuclei

Answer 14

nuclei with magnetic moments that are aligned with the magnetic field they’ve been placed in; they are lower energy in NMR

Question 15

beta-state nuclei

Answer 15

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

Question 16

How NMR spectrum works

Answer 16

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

Question 17

^1H-NMR

Answer 17

NMR studies absorption of protons and only ^1H are studied on the mcat (hydrogen)

Question 18

chemically equivalent

Answer 18

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

Question 19

integration

Answer 19

area under the peak of an H-NMR spectra; size of peak is proportional to number of protons it reflects

Question 20

deshielding

Answer 20

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

Question 21

spin-spin coupling (splitting)

Answer 21

occurs when two or more hydrogens are three bonds away from each other like H-C-C-H

Question 22

doublet

Answer 22

number of peaks that occur in H-NMR when two hydrogens are in close proximity (not attached to O or N)

Question 23

n + 1 rule

Answer 23

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

Question 24

coupling constant, J

Answer 24

magnitude of splitting in H-NMR

Question 25

main shifts to know on H-NMR

Answer 25

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