Organic Chemistry Spectroscopy Flashcards
___ is the process of measuring the frequencies of electromagnetic radiation (light) absorbed and emitted by a molecule
spectroscopy
___ measures molecular vibrations
infrared spectroscopy
The ___ of a sample is determined by passing infrared light through the sample and recording the frequencies at which the light is absorbed. The useful absorptions of infrared light occur in the frequency range 4000 - 400 cm-1 (called wavenumbers), which can be split into a functional group region and a fingerprint region. In the functional group region, the wavenumber of the absorption is typically related to the stretching vibrations of specific functional groups in a molecule and can be used to identify these functional groups. In the fingerprint region, the spectra, which usually consists of bending vibrations within a molecule, is complex and unique (like a fingerprint) for each compound.
infrared spectrum
___ is observed in the region of 4000-1500 cm-1, and bending vibrations are observed in the region of 1500-400 cm-1
bond stretching
In order for IR light to be ___, the vibration of the molecule must result in a change in a bond dipole moment. Symmetrical molecules do not have a dipole moment and therefore do not absorb IR light. The more complicated a molecule, the more peaks observed in its IR spectrum
absorbed
Alkanes 2800-3000 C-H 1200 C-C Alkenes 3080-3140 ||C-H 1645 C||C Alkynes 3300 |||C-H 2200 C|||C Aromatic 2900-3100 C-H 1475-1625 C-C Alcohols 3100-3500 O-H (broad) Ethers 1050-1150 C-O Aldehydes 2700-2900 (O)C-H 1725-1750 C||O Ketones 1700-1750 C||O Acids 2900-3300 O-H (broad) 1700-1750 C||O Amines 3100-3500 N-H (sharp)
Some of the most useful information in an ___ can often be quickly and easily determined by reviewing two key areas of absorption - from 1700-1750 and 3100-3500 cm-1 - to determine the presence of oxygen and nitrogen functional groups
Functional Group 1700-1750 cm-1 3100-3500 cm-1 Alcohol No Broad peak Aldehyde or Ketone Yes No Carboxylic Acid Yes Broad Peak Amine No Sharp Peak
IR spectrum
A great deal of information can be obtained from an IR spectrum. ___, the inverse of absorbance, is plotted versus frequency; this gives areas of absorption the appearance of valleys on the spectrum
percent transmission
___ is measured in the frequency range of 4000-400 cm-1, and is plotted as transmittance vs. wave number, with absorption appearing as “valleys” in the spectrum.
Different functional groups show IR absorption at different wave numbers. Some of the most useful absorption ranges to note are at 1700 cm-1 (C||O bond) and at 3100-3500 cm-1 (O-H if broad, N-H if narrow). However, other absorption frequencies an also be important
IR absorption
Certain ___ have magnetic moments that are normally oriented at random. When such nuclei are placed in a magnetic field., their magnetic moments tend to align either with or against the direction of the field. Nuclei whose magnetic moments are aligned with the field are said to be in the alpha-state (lower energy, while those whose moments are aligned against the field are said to be in the beta-state (higher energy)
nuclei
If the nuclei are then exposed to ___, some in the lower energy alpha-state will be excited into the beta-state; this transition is known as resonating. The absorption corresponding to this excitation occurs at different frequencies depending on the atom’s environment. Other nearby atoms in the same molecule (or sometimes in the solvent) can shift the frequency at which an atom transitions between the alpha and beta states. A compound may contain many nuclei that resonate at different frequencies, producing a very complex spectrum
electromagnetic radiation
Since different NMR ___ operate at different magnetic field strengths, a standardized method of plotting the NMR spectrum has ben adopted. A variable called chemical shift, represented by the symbol lowercase delta and with units of parts per million (ppm), is plotted on the x-axis. Chemical shift is compared to the standard organic molecule tetramethylsilane (TMS). TMS is always shown by a peak at 0 ppm, and the peaks of interest always appear to the left of TMS. A peak further to the left is said to be shifted downfield
spectrometers
Most H,1 nuclei come into ___ between 0 and 10 delta downfield from TMS. each distinct set of nuclei gives rise to a separate peak. The compound dichloromethyl methyl ether(Cl2HC-O-CH3) has two distinct sets of H,1 nuclei. The single proton attached to the dichloromethyl group is in a different magnetic environment compared to the three protons on the methyl group, and the two types of protons resonate at different frequencies. The three protons on the methyl group are magnetically equivalent, due to rotation about the oxygen-carbon single bond, and resonate at the same frequency.
resonance
If two magnetically different ___ are within three bonds of each other a phenomenon known as coupling or splitting occurs. Consider two protons, H sub a and H sub b, on the molecule 1,1,-dibtomo-2,2-dichloroethane.
At any given time, H sub a can experience two different magnetic environments, since H sub b can be in either the alpha or the beta state. These different states of H sub b influence nucleus H sub a, causing slight upfield and downfield shifts. Since there is a 50 percent chance that H sub b will be in either state, this results in a doublet, two peaks of equal intensity equally spaced around the true chemical shift of H sub a. H sub b experiences the two different states of H sub a an dis likewise split into a doublet. In 1,1-dibromo-2-chloroethane, the H sub a nucleus is affected by two nearby H sub b nuclei, which, taken together, can be in four different states: alpha-alpha, alpha-beta, beta-alpha, or beta-beta.
The alpha-beta and beta-alpha states have the same net effect on the H sub a nucleus, and the resonances occur at the same frequency. The alpha-alpha and beta-beta states resonate at frequencies different from each other and from the alpha-beta/beta-alpha frequency. The result is three peaks centered around the true chemical shift, with an area ratio of 1:2:1. The number of peaks that will appear when splitting occurs can be determined as follows: n hydrogen atoms couple to give n+1 peaks, whose area ratios are given by Pascal’s triangle.
protons
C, 13 NMR is very similar to H, 1 NMR. Most C, 13 NMR signals, however, occur 0-200 delta downfield from the carbon peak of TMS, so a spectrum for C, 13 NMR is easily recognized by the larger scale of the x-axis. Another significant difference is that only 1.1 percent of carbon atoms are C, 13 atoms. This has two effects: First, a much larger sample is needed to run a C, 13 spectrum (about 50 mg compared with 1 mg for H, 1 NMR), and second, coupling between carbon atoms is generally not observed.
___ is observed, however, between carbon atoms and the protons directly attached to them. If a carbon atom is attached to two protons, it can experience four different states of those protons (alpha-alpha, alpha-beta, beta-alpha, and beta-beta), and the carbon signal is split into a triplet with the area ratio 1:2:1. An additional feature of C, 13 NMR is the ability to record a spectrum without the coupling of adjacent protons. This is called spin decoupling, and produces a spectrum of singlets, each corresponding to a separate magnetically equivalent carbon atom. Compare the following two spectra, both of 1,1,2-trichloropropane. One is a typical spin-decoupled spectrum, and the other is spin-coupled
coupling
In general, ___ provides information about the carbon and hydrogen skeleton of a molecule along with some suggestion of its functional groups
NMR spectroscopy
