Spectroscopy Flashcards
define degree of unsaturation
the number of pi bonds and/or rings in a molecule
give the formula for degree of unsaturation
[(2C + 2) - H + Group 5 - Group 7]/2
function of Mass Spectrometry
gives information about the mass of a compound and the fragments from which it is formed
describe how a typical mass spectrometer functions
- molecules are ionised (made into charged species) and fragmented
- ions are separated based on mass depending on their mass-to-charge ratio
what does a mass spectrum show?
- 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
define spectroscopy
the measurement of the interaction between a molecule and electromagnetic radiation
what is the effect of infrared radiation?
it causes excited stretching and bending vibrations (oscillations) of bonds that contain a dipole (no dipole = no vibration)
four types of stretching caused by IR
- symmetrical stretching
- asymmetrical stretching
- in-plane bending
- out-of-plane bending
describe how frequency of vibration is related to bond strength and atom weight
stronger bonds and lighter atoms vibrate at higher frequencies (higher energies)
draw a diagram for an IR spectrometer
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
describe the appearance of a typical IR spectrum
different bonds vibration at different energies which give rise to unique absorption bands with characteristic intensity, shape and frequency, stated as wavenumber
state the 4 main regions of a typical IR spectrum
from left to right:
- hydrogen region
- triple bonds
- double bonds
- fingerprint region (less useful)
energy and frequency increase from right to left
IR spectra: alkanes primarily give
C-H stretches (sp3)
IR spectra: alkenes primarily give
C-H stretches (sp2) and C=C stretches
IR spectra: alkynes primarily give
C-H stretches (sp) and C≡C stretches
IR spectra: aromatics primarily give
C-H stretches (sp2) and C=C stretches
what is a defining characteristic of Oh bands and NH bands
they are broad at 2700 - 3600 cm-1
are OH or NH bands stronger
OH bands are broader (more H bonding is occurring) and stronger than NH bonds (the more polar the bond, the stronger the signal)
carbonyl groups produce
strong and sharp C=O bands
how does NMR work?
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
3 steps by which NMR works
- the population of nuclei in the lower (more stable state) is slightly greater
- 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
- as the nuclei relax back, they emit a signal that provides information about their unique chemical environment
magnetic field strength of small organic molecules vs large biomolecules
small: 300-700 MHz
large: 700+ MHz
what information does a HNMR spectrum provide ?
- hydrogen types: the number of signals shows the number of hydrogen environments
- integration: the peak area for each hydrogen type gives the number of H associated with each hydrogen type
- 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
chemically equivalent hydrogens have:
- 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
how does shielding impact chemical shift?
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)
how does electronegativity impact chemical shift?
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)
how does magnetic anisotropy affect chemical shift?
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
how does hydrogen bonding affect chemical shift?
hydrogen atoms on heteroatoms (OH and NH) are usually broad signals that have variable chemical shifts due to H bonding
