Dimensions

Each axis of an NMRData has a dimension type, which says what the axis measures. The type determines how processing functions treat the axis, how plots label it, and which selectors make sense on it.

The dimension types

NMRDimension
├── FrequencyDimension          F1Dim  F2Dim  F3Dim  F4Dim
└── NonFrequencyDimension
    ├── TimeDimension           T1Dim  T2Dim  T3Dim  T4Dim   (frequency evolution)
    │                           TrelaxDim                    (relaxation delays)
    │                           TkinDim                      (real time, kinetics)
    ├── GradientDimension       G1Dim  G2Dim  G3Dim  G4Dim   (diffusion)
    ├── OffsetDimension         OffsetDim                    (CEST and other offsets)
    ├── FieldDimension          SpinlockDim                  (spin-lock fields)
    └── UnknownDimension        X1Dim  X2Dim  X3Dim  X4Dim   (anything else)

• Frequency dimensions (F1Dim–F4Dim) have chemical shift coordinates in ppm. They are produced by Fourier transformation, or loaded directly from processed data.
• Time dimensions for frequency evolution (T1Dim–T4Dim) have coordinates in seconds. They are what loadfid returns, and what ft turns into frequency dimensions.
• Other non-frequency dimensions describe the extra axis of a pseudo-2D or pseudo-3D series: relaxation delays, kinetic time points, gradient strengths, offsets or spin-lock fields. They are never Fourier transformed.
• Unknown dimensions (X1Dim–X4Dim) are non-frequency axes whose meaning NMRTools could not work out from the acquisition parameters. A pseudo-2D relaxation series is loaded with an X2Dim, and you replace it with a TrelaxDim once you know the delays.

The number in a name gives the order of the dimension in the data: F1Dim (or T1Dim) is the direct dimension, F2Dim the first indirect dimension, and so on.

From time to frequency

ft replaces each TnDim with the matching FnDim, keeping its metadata. Non-Fourier dimensions are left alone. For a 2D experiment:

loadfid   →  (T1Dim, T2Dim)       time domain, Multicomplex{Float64,2,4}
ft        →  (F1Dim, F2Dim)       frequency domain, Multicomplex{Float64,2,4}
realest   →  (F1Dim, F2Dim)       frequency domain, Float64

and for a pseudo-2D relaxation series:

loadfid         →  (T1Dim, X2Dim)
setrelaxtimes   →  (T1Dim, TrelaxDim)
ft              →  (F1Dim, TrelaxDim)

Processing functions use the dimension types to decide what to act on: called without a dimension, apodize, zerofill and ft act on every TimeDimension that has an imaginary unit, and skip the rest.

Referring to a dimension

Functions that take a dimension accept its type (F1Dim), its number (1), or, for frequency dimensions, its nucleus (H1, N15). resolvedim converts any of these to a number and finddim finds the dimension of a given nucleus:

using NMRTools
spec = exampledata("2D_HN")
finddim(spec, N15)

2

Changing a dimension

replacedimension swaps one dimension for another of a different type with new coordinates. The set* functions do this for the common cases:

shiftdim moves the coordinates of a frequency dimension by a fixed number of ppm.

Related pages

• Multicomplex numbers and quadrature: how each time and frequency dimension is linked to its imaginary unit
• Process step by step: processing pseudo-2D data