Spectroscopy

Question 1

define degree of unsaturation

Answer 1

the number of pi bonds and/or rings in a molecule

Question 2

give the formula for degree of unsaturation

Answer 2

[(2C + 2) - H + Group 5 - Group 7]/2

Question 3

function of Mass Spectrometry

Answer 3

gives information about the mass of a compound and the fragments from which it is formed

Question 4

describe how a typical mass spectrometer functions

Answer 4

• molecules are ionised (made into charged species) and fragmented
• ions are separated based on mass depending on their mass-to-charge ratio

Question 5

what does a mass spectrum show?

Answer 5

• detected ion masses (m/z) and their relative abundance
• most important peak is the molecular ion peak (usually the heaviest ion in large abundance, M+) as it represents the molecular mass of the compound

Question 6

define spectroscopy

Answer 6

the measurement of the interaction between a molecule and electromagnetic radiation

Question 7

what is the effect of infrared radiation?

Answer 7

it causes excited stretching and bending vibrations (oscillations) of bonds that contain a dipole (no dipole = no vibration)

Question 8

four types of stretching caused by IR

Answer 8

• symmetrical stretching
• asymmetrical stretching
• in-plane bending
• out-of-plane bending

Question 9

describe how frequency of vibration is related to bond strength and atom weight

Answer 9

stronger bonds and lighter atoms vibrate at higher frequencies (higher energies)

Question 10

draw a diagram for an IR spectrometer

Answer 10

consists of a high-quality infrared light source, a slit to create a parallel beam, a sample carrier, and a detector
- there is an incident IR light beam
- light is absorbed by the compound only at its vibration frequencies
- IR light absorbed by the compound is absent at the detector, showing up as IR peaks

Question 11

describe the appearance of a typical IR spectrum

Answer 11

different bonds vibration at different energies which give rise to unique absorption bands with characteristic intensity, shape and frequency, stated as wavenumber

Question 12

state the 4 main regions of a typical IR spectrum

Answer 12

from left to right:
- hydrogen region
- triple bonds
- double bonds
- fingerprint region (less useful)
energy and frequency increase from right to left

Question 13

IR spectra: alkanes primarily give

Answer 13

C-H stretches (sp3)

Question 14

IR spectra: alkenes primarily give

Answer 14

C-H stretches (sp2) and C=C stretches

Question 15

IR spectra: alkynes primarily give

Answer 15

C-H stretches (sp) and C≡C stretches

Question 16

IR spectra: aromatics primarily give

Answer 16

C-H stretches (sp2) and C=C stretches

Question 17

what is a defining characteristic of Oh bands and NH bands

Answer 17

they are broad at 2700 - 3600 cm-1

Question 18

are OH or NH bands stronger

Answer 18

OH bands are broader (more H bonding is occurring) and stronger than NH bonds (the more polar the bond, the stronger the signal)

Question 19

carbonyl groups produce

Answer 19

strong and sharp C=O bands

Question 20

how does NMR work?

Answer 20

some atomic nuclei, like protons (1H), behave like spinning spheres. since the nuclei (positively charged) have electrons (negatively charged) surrounding then, a small local magnetic field is created when they spin (magnetic moment).
- in the absence of an external magnetic field, the magnetic moments are randomly oriented
- when an external magnetic field (B0) is applied, the magnetic moments align, with some opposed to and some parallel to B0

Question 21

3 steps by which NMR works

Answer 21

1. the population of nuclei in the lower (more stable state) is slightly greater
2. electromagnetic radiation causes some nuclei to become excited from a lower to higher E state (spin-flip) the frequency at which this spin flip occurs is called the resonance frequency
3. as the nuclei relax back, they emit a signal that provides information about their unique chemical environment

Question 22

magnetic field strength of small organic molecules vs large biomolecules

Answer 22

small: 300-700 MHz
large: 700+ MHz

Question 23

what information does a HNMR spectrum provide ?

Answer 23

1. hydrogen types: the number of signals shows the number of hydrogen environments
2. integration: the peak area for each hydrogen type gives the number of H associated with each hydrogen type
3. chemical shift: the position on x axis of each signal gives the electronic environment of each proton type. this includes hybridisation of attached carbon, presence of adjacent functional groups, etc

Question 24

chemically equivalent hydrogens have:

Answer 24

• identical environments (interchangeable by bond rotation or a plane of symmetry)
• identical chemical shifts (share the same signal) number of hydrogen types = number of signals

Question 25

how does shielding impact chemical shift?

Answer 25

the electrons around a nucleus create a magnetic field opposing the applied field. this reduces the apparent field, thereby shielding it from the applied magnetic field. - shielded = more electron rich = upfield (lower chemical shift) - deshielded = more electron poor = downfield (higher chemical shift)

Question 26

how does electronegativity impact chemical shift?

Answer 26

electronegative atoms deshield and shift protons towards the left: higher radio frequency (more energy) needed for proton resonance - increasing electronegativity means less electron density around H - the electronegativity effect is roughly additive and depends on proximity (inductive effect diminshes with distance)

Question 27

how does magnetic anisotropy affect chemical shift?

Answer 27

pi-electrons generate a local diamagnetic current that opposes the applied magnetic field (B0). This causes a shielding cone where nuclei inside the cone are shielded (smaller ppm) and nuclei outside the cone are deshielded (larger ppm) - causes hydrogen atoms attached to pi systems to have a dramatic deshielding effect compared to alkane H's - adjacent pi systems also have a small deshielding effect

Question 28

how does hydrogen bonding affect chemical shift?

Answer 28

hydrogen atoms on heteroatoms (OH and NH) are usually broad signals that have variable chemical shifts due to H bonding